Inconsistent mail time sending DVDs back to Netflix

Two weeks in a row now, Netflix has shown a longer than normal delay in receiving my movies. I usually mail my movies back from work in zip code 55113. Whereas Bonnie finishes her movie, and sends back from our home mailbox in zip code 55444. This week, I mailed my movie on Monday and it did not appear until Thursday as received by Netflix. Last week, I also mailed on Monday, and they didn't see it arrive until Friday. Yet Bonnie's movies have showed as received the following day. So last week, we both sent movies back on Monday (from different zips), hers arrived to NF on Tues, she had her next movie arrive in the mail Weds, all the while mine didn't show arrived at NF until Friday. What gives?