This is a fascinating topic. I am hoping for a great discussion. Thanks to the OP for a great question.
In short - yes, there are multiple warfare AI designs in use in the game, and they all handle your army and attack and your play strategy differently. It's sometimes subtle (which is what INNO has hoped for - gradual strangling...), and sometimes not.
The AI for: Continent Map, GbG, PvP, GE - etc - ALL react differently to your various strength's, matchups, injury, battle decision, and predatory nature. It's extremely complex behind the covers, and uses a lot of probability to decide things. As you get wounded, at least in GbG, Inno has "amped" up the AI to become much more predatory to your wounded soldiers over the last 2 years of GBG.
All quietly of course, and it fits within their theme to "slow you down" in GbG.
Inno has quietly and somewhat stealthily added lots of power to the defense in GbG, specifically. In a word, they are all freshly "adaptive". It would be a great intellectual exercise to test this - for example, if you could take your same army, against the same defense, on the same terrain, in each of the venues. But we can't.
You can get the "texture" of your contention from playing in each of these venues (GE, GBG, GvG, etc.) pretty much as you've done though, and your assessment is spot on, IMHO.
As mentioned, you add attack and defense % boosts via event buildings, or potions, or other game items, but INNO's defense algorithm matches what you add with quiet reductions. It's as if you take a few steps forward, but the AI recognizes that, and retards some of the power you've purchased or obtained for your city. If your army is @ 1000 % attack, adding 18% in an event building seems like you've made a decent gain for 20 squares of spaces, but in reality - you can't feel it.
Mathematically, you get value. Theoretically, you get value. But practically? You're going to say - I see no difference.
In fact, add an "on demand" 50% attack boost (tavern boost + inventory attack potion), and you can barely see that in your results either, especially in mid-range ages or senior game ages. Why?
The AI will adapt to that against you. Balance. The game will not give you the upper hand.
No one's gonna tell you that of course, but they have. With people changing their cities to be pure attack, if they left the AI "the way it was when we started GbG and made no changes to allow it to float adaptively to anything you'd try to be able to BEST the game experience, the true cyborg armies that people have created would easily overrun the game...
and we can't have that.
The war AI (especially in GbG) is much smarter intrinsically. It's more predatory, intrinsically and... it really battles adaptively - they study what you're sending against them, and react to give you an experience so that you can say - that SHOULDN'T be happening.
But it is.