receipt date is the date when the mail room receives your application. Notice date is when the application is being entered into the computer and the receipt notice is generated.
If both of you sent and mailed your paperwork on same date, then you should have the same receipt date. I noticed sometimes that INS inputs the data based on the same type of paperwork.
they input all I-485 together, then all I-131 together, I-90, and so forth...
Also the notice date corresponds to the number of day of the year. Example...02-220...represents, year 2002, and day 220, which is 06/18. 02-221, year 2002, and day 221, which is 06/19 and so on...
I hope this clarifies it.