Thanks @GhostLegion for pointing this out 
Top Cover:
I also assume that the 11th gen version has already been built with the old cover and that they are just selling the remaining stock but I can also imagine that they changed the cover at some point (maybe they had more mainboards than cases and are now selling the old mainboard with newer cases). But I wasn’t able to find a definitive answer to that question.
Since the new top cover seems to have a slightly higher weight (25g according to this review), I expected the 12th gen version to be that much heavier if the 11th gen would not have the new top cover, but it turns out that the 12th generation is almost 700g hevier (1.3 kg for the 11th gen vs 4.40 lbs (~ 2 kg) for the 12th gen) so that was a dead end.
Hinge:
I have not got an official answer regarding the top cover but @amoun already quoted an official answer regarding the hinge:
The 4.0kg hinge is not meant as an upgrade but targets users that have to use the laptop in “bumpy” environments like a car or a bus. The 3.3 kg hinge is designed to make it possible to open the laptop with one hand (that is without holding down the bottom part while folding it open) but if the laptop is moved heavily while it is open, the hinges might not be strong enough to hold it in place and thus, the display might “fall down” (it opens up wider).
There was also a time when the 3.3 kg hinge was manufactured poorly and did not actually hold 3.3 kg so displays did fall down even when standing still. But this issue has since been resolved and even the 11th gen comes with a hinge that does not fall down now.
Since I posted my question I have decided to buy the 12th gen version anyways so I still don’t know the answer to the top cover question, so @dragan feel free to contact support if you are still interested in the answer. If you post a definite answer from the support team, I’ll mark it as the solution.