As I have been scanning negatives for a while recently, I can offer my own insight here. In many cases, the 135 and 110 format negatives I've been scanning to not have even tone across the entire surface of the frame. With the larger format, there is often a pronounced "vignette" at the edges. For both formats sometimes the film just doesn't lay flat, but in a regular manner, leading to one side or the other often having a lighter or darker grade than the center or other side of the frame.
To combat this issue, I've resorted to cropping all the scanned frames in an identical way. Now, I chose to keep things in the 3:2 ratio, so I am dropping the top and bottom more than I actually need to in order to crop out the unsightly left and right sides. It is possible that the lab you used to develop and digitize your film follows a similar procedure, however in their case they chose not to maintain the aspect ratio.
This is all speculation, honestly, but given my own recent experience, I thought I'd offer the possibility.