You indeed noticed something interesting so I decided to take a look and... it appears you are quite right mate. The BATTLE difficulties very hard and easy are inverted.
I just made a simple mod that changed the bonuses the AI gets on very hard battle difficulty. Vanilla bonuses are:
- Charge bonus +15%
- Melee attack +15%
- Melee damage +15%
- Melee defense +20%
I changed those values to ridiculous levels:
- Charge bonus +500%
- Melee attack +500%
- Melee damage +500%
- Melee defense +500%
I started a new Empire campaign using my mod with the above changes. I attacked the Secessionists using Karl Franz and his starting army. I checked that the battle difficulty was very hard. I had an easy win, losing 35% of Franz health and only 23 men. If the bonuses were taking into account it would have been a slaughter of my units.
I reloaded the game this time with the easy battle difficulty. The battle was completely different, it wasn't a battle at all but a massacre of my men due to the insane bonuses in my mod.
Conclusion: easy battle difficulty IS in fact very hard and very hard IS the easy one.