There is a definite difference in difficulty of encounters within each province, at least in the early provinces and early rounds, which is what I can best speak to.
Digression: This is perhaps less noticeable to people who manually fight and more noticeable to people who auto fight. I auto fight everything. If I wanted to control lots of units going after each other, I'd be playing an FPS. (Nothing against people who manually fight. I'm glad that's an option for them, but I personally am not interested, and hope the game continues to accommodate people like me, such as by offering the option to cater these difficult fights.)
Back to my main point: In each province there is usually one encounter at least (sometimes two) that feature an "impossible" combination of shady characters. That is, it's impossible to select one opposing troop type that is reasonably effective (at least even) against all of them. From my observation of the way Auto Fight works, it is most effective in terms of limiting losses when all of your deployed troops are of the same type. Sure you can TRY to throw some treants in front of your archers in order for them to take the hits, but the Auto Fight is not as efficient as manual fight at employing relatively complex tactics. I don't know how the math is done, but in my observation in general losses are better when you Auto Fight with just all one troop type. The "difficult" provinces with mixes of troops make it difficult to do this, resulting in larger proportional losses.
I'm a new player and have only unlocked 4 hexes at most for each tournament, and I haven't fought them beyond the fourth round, so what I'm describing here may change as things progress. Perhaps in later provinces or rounds everything is equally hard. If that's the case, I think that supports my argument even more for not combining everything into one big fight in these earlier provinces and/or rounds.