Well there is a visible difference in the seat construction from Airbus to Boeing ac, but that doesn't answer your question.

Seatguru.com says that the 757 seats are narrower by 1" and have 1" more of pitch versus the 777 and 767, but that doesn't explain the differences in padding.

From experience, I know that airlines contract with different aircraft interior "outfitters". I thought they had a vendor or two that they worked with over the years, but perhaps they have moved from one interior outfitter to another. Or they may have tried different grades of foam over the years. I suspect they purchased the 767s, 777s and 757s at different points in time and therefore they ordered them with different foam specs or the manufacturer slipped the different foam into a general spec. I know the outfitter that I did some consulting for, they were located just north of Dallas, had all the AA contracts, but they were working on the LH 777s and other foreign based airlines. The seats were to different specs that each airline chose.

Again this is all just speculation and I haven't been in direct contact with this customer in over 10 years. Sorry this may be dated info.

FB