That would make sense. It should be fairly safe to assume that the smaller seats would be the exceptions (they aren’t randomly going to make one seat wider just because they have space but may make some narrower to accommodate tray tables in armrests).

I also swear that in some exit rows where the exit door protrudes, the window seat itself feels narrower (thinking the C+ exit row 13A on A321). Am I just imagining that?

I also can imagine that these published dimensions reflect the maximum width without protruding into your neighbor’s space. If you are over by a little bit and it encroaches into your seat, it might not be a big deal (eg the seat will still fit but might just make your seat a little tighter).