So here's my impression of this problem after getting my feet scanned.
I currently wear a 7.5D Jetspeed FT1 retail...after getting my foot scanned, the system recommended a size 8D RibCor or 8D Jetspeed. However if you look at the 2nd page of the scan, which measures your foot in CM, it showed my left foot at 26.5 CM (size 7.5) , and my right foot at 26.9 CM (size 8) So I think for the recommendation they go off of the bigger foot so as not to have one foot smashed, since the pro custom (which shapes your mold around a retail boot) will have both feet the same size. If you go full custom it allows you to get a different length size for each boot in which case CCM will use the actual CM measurements of each foot