Hakan-i Cihan
Emperor
Everyone seem to find ratios more natural, but differences are easier to calculate. maybe developers are trying to increase customer base with some boardgame players, I'm not sure.
Differences are easier to calculate, yes. But a +7 (I Battlecry: +7 when attacking melee and ranged units) bonus will be very different for a warrior with melee strength of 20 in comparison to infantry with melee strength of 70. For warrior it is like 35% addition to its strength, while for the infantry it is like 10% addition. For the mechanized infantry it will be even way less.
Maybe the reason for this is to counter the players promoted units. Players can keep their units with a lots of promotions alive, while the AI isn't able to do this properly. So devs could have thought to counter this lack of AI by making promotions less effective for later units by making the bonuses absolute values instead of relative.