If I look at my itineraries, MSP-CDG is 4206 miles while CDG-MSP is 4207. The actual number that DL used when posting to my account was 4222.

However, my favorite example (so far) is MSP-HND at 5932 miles but HND-MSP is 5950. Actual was 5982.

On an itinerary, ATL-EZE is 5015 while EZE-ATL is 4999, so the hypothesis that DL thinks the trip back home is longer doesn't work. Last year DL thought it was 4999 each way.