You are confused about the DIFFERENT naturalization eligibility requirements and greencard maintenance requirements. You probably will be applying for naturalization based on at least 5 years as a greencardholder (most people do apply on that basis under INA 316(a)).
1.) Length of time with a greencard. At least 5 years. So, USCIS will look at the 5 years immediately before the N-400 is filed for MOST requirements.
******This 5 years is your basic statutory period (other things before then can have an effect but they are
oddball things like Murder, rape, "aggravated felonies" or "Communist party membership" or genocide and persecution of others.)
******For in depth information on disruption of residence for naturalization purposes, please read 8 CFR 316.5(c)(1) completely, see
www.uscis.gov
******Aside from naturalization eligibility, one must be concerned with maintaining their underlying immigrant status. A trip of 6 months or more will be viewed as a potential sign of abandonment of status. A period of a year or more is a presumptive abandonment. An absence of a year or more without a re-entry permit means that you would need to apply for a Returning Resident Visa (SB-1), or refugee travel document if a refugee or asylee, before being allowed back in. If an LPR knows that they may need to be abroad for a long period and applies for a re-entry permit in advance of departure, they may safely stay abroad for up to 2 years and return without being viewed as having abandoned greencard status. However, any LPR who has been abroad for over 180 days is seeking an admission into the U.S., even with a re-entry permit so needs to avoid becoming inadmissible. (i.e., don't get arrested or engage in prohibited activities)
INA 101(a) [8 USC 1101(a)] [this section contains "definitions", below excerpt is referring to 8 USC sections]
(13)(A) The terms ``admission'' and ``admitted'' mean, with respect
to an alien, the lawful entry of the alien into the United States after
inspection and authorization by an immigration officer.
(B) An alien who is paroled under section 1182(d)(5) of this title
or permitted to land temporarily as an alien crewman shall not be
considered to have been admitted.
(C) An
alien lawfully admitted for permanent residence in the United
States shall not be regarded as seeking an admission into the United
States for purposes of the immigration laws unless the alien--
(i) has abandoned or relinquished that status,
(ii) has been absent from the United States for a continuous
period in excess of 180 days,
(iii) has engaged in illegal activity after having departed the
United States,
(iv) has departed from the United States while under legal
process seeking removal of the alien from the United States,
including removal proceedings under this chapter and extradition
proceedings,
(v) has committed an offense identified in section 1182(a)(2) of
this title, unless since such offense the alien has been granted
relief under section 1182(h) or 1229b(a) of this title, or
(vi) is attempting to enter at a time or place other than as
designated by immigration officers or has not been admitted to the
United States after inspection and authorization by an immigration
officer.
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2.) Physical presence inside the U.S. For this one requirement they will do the simple math and you must show that you have been inside the U.S. for at HALF of your statutory period.
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3.) Continuous Residence in the U.S. This refers to your qualitative connection to the American society that you wish to join as a citizen. A trip of 6 month can be viewed as disruptive of the continuity of residence for naturalization purposes. A trip of a year or more will be viewed as disruptive of the continuity of residence for naturalization purposes EXCEPT in a few oddball situations that you have not indicated any connection to so I won't go into them.
The origin of the continuous residence requirement dates back to the Naturalization Act of 1790, which at that time required 2 years residence. The idea was, and remains, to allow a person to become "Americanized" or join into the society in which they desire citizenship and will enjoy the right to vote or serve on juries or hold any but the highest office. This residence has always been tied to the period immediately before naturalization in an effort to have a person enter the society as it currently exists.
If a person gets a greencard and immediately, or shortly thereafter, returns home for a long period with only a few "visits" to the U.S. to maintain LPR status, then FINALLY comes to live in the U.S., the mere fact of having a greencard for a long enough period of time does not prepare that person to join the American society as it currently exists. They are strangers in a strange land and unprepared to become part of "We the People..."
CONCLUSION: The disruption of continuity of residence can have the overarching effect of pushing off your N-400 filing date thus changing the five year statutory period and the time during which physical presence can be calculated. There are different requirements to be considered individually but they are ultimately interconnected.
