I485 case number, mine and my husband's should be different?

NIWEAer

Registered Users (C)
My husband is the principle applicant, and my I485 filed a little later than his. I got different I485 case number with his. Is this correct? USCIS did not treat as two different cases?
 
Thanks.
I was a little bit worry because the cover letter for my I485 was mailed in separate envelope (not with my I485) to USCIS. Hopefully they have known I want to put my I485 with my husband's.
 
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