Yes! The difficulty is the problem! Here are the results from a test I ran for another thread:


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A few experiments (all battles fought until 1 side routs, large battle, syrian flats during winter, no weather):

first number is my troops left at rout time, second is theirs, lower number always represents the loser

Hastati vs Hastati:

easy level:
(the AI never threw pila on easy, but my troops did)
61/10
65/32
60/13
66/15
62/14

medium:
35/6
13/21
13/54 (my general died early)
18/2
5/22

hard:
6/41
5/52
2/39
3/52
4/48

very hard:
(on very hard, my troops broke and ran much earlier than on hard, which is why there are more of them left at rout time)
17/64
11/57
18/58
11/66
24/61

Next I tried Cataphracts (the regular horse type).

Again the first number is my guys left, the second is the AI's, and each side starts with 54 horsemen.

Easy:
45/11
44/12
42/8
47/12
47/18

Medium:
30/20
27/18
5/29
1/18
24/11

Hard:
1/32
16/52
3/45
18/50
15/48

Very Hard:
(again, like with the Hastati, my men broke much quicker than on Medium)
22/47
19/49
20/50
19/47
17/48
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As you can see, the AI gets a huge bonus to combat in the hard/very hard level. In fact, the event that made me go research this in the first place was playing the Seleucid empire, and having the AI walk right through my spearwalls like they didn't exist. If you want a fair fight (unit vs unit that is), you're forced to play Medium, with any resultant tactical AI deficiencies that brings in.

Difficulty levels work differently in RTW than MTW/STW it seems.