Indeed, this is an old and known knowledge.
However, IIRC this applieds to upgrade of armour, not to the base value. So if:
- unit A has initial armour of 3,
- unit B has initial armour of 5,
and then
- unit A upgrades the armour stat by 2,
- unit B upgrades the armour stat by 1,
the result is:
- unit A has initial armour of 8,
- unit B has initial armour of 7.5.
@Gigantus, do you concur?