I've researched this topic a lot and there seems to be 4 schools of thought on how armor upgrades work in M2TW and by extension EBII
1. Each upgrade adds 2.5 armor (EBII "official" answer)
2. Each level adds an individual bonus between 1 and 4 depending on the level
3. Each level adds a cumulative bonus of all levels' bonuses all the way up to that level
4. Each level sets the armor to the cumulative amount
#2-4 were theorized by players in 2007, while #1 was answered by a player in 2017, so I'm inclined to believe the more up-to-date answer. But for anyone with advanced knowledge of M2TW internals, which explanation is correct?