So then it explains why you see no difference, i had 2x 2017 replacements and they were the same, this last one that is dated 8th March 2018 it feels different. The other user that confirmed his replacement feels different is also a 2018.
It's the age of the battery however, unlikely this will also be the age of the keyboard as they are separate components. Yes they are replaced and installed together, however they will not be stored together. A lot of people were sure the 2017 keyboard felt different to the 2016 keyboard when the 2017 first came out. I've always just put this down to a softening of the keys from new, and a confirmation bias that something was fixed.
Either way I wouldn't read too much into these things, it'll always be subjective. My 2017 keyboard does on the surface feel a little different to the previous one, however they both feel exactly the same if comparing it to when it was new.