People are confused with Visa vs Duration of Stay.
A US Visa stamped oversees only entitles one to enter the US before its expiration. It does not guarantee authomatic entry to the US because that is left to the Immigration Officer at the POE. Its the date stamped by the Immigration Officer in the I-94 at the POE that determines how long one can stay.
A person with a 1 month visa can receive 6 months at the POE. Thats a blessing. A reverse can and has happened many a times. For eg: folks with 10-yr visa can only stay 6 months at a time, again based on the I-94 stamp.
I know of a person, who received a 3-month tourist visa from Madras and a 6-month on his I-94 at JFK, faced problem at Madras 5 yrs later when he applied again for a visit visa. He had to explain the fact that he was given 6 months at the POE. Sometimes, the embassy folks don't like POE Iimigration Officers authorizing a stay longer than what the embassy intends.
In either case, one can stay upto the date stamped on the I-94 at the POE, regardless of the visa duration. Visa is just for travel and I-94 determines the stay.
Hope this info helps.