Ok, I've looked into this now, and it's quite interesting. A couple of factors are causing this:
1) When selecting the best unit it calculates odds for each unit in the stack in turn. Odds are restricted to 1-99% (100 and 0 both cause issues with rounding, so we discard them and treat as 99 and 1 respectively). If a unit has better odds than the one previously checked it is noted as the best so far, and the eventual best is returned.
2) If several units have odds of 99% or above then the first one evaluated will 'win'. In your case, for any animal attack, and also for an attack against a Neanderthal warrior (at least in non-defensive terrain - I didn't check against a Neanderthal on a wooded hill), the clubmen has odds of over 99% (which is not what is displayed in the hover text, and I'll come to that) and happens to be the first unit in though group (so it gets evaluated first). Since the two spearmen also get a 99% evaluation they do no replace the clubman, and it gets used because all have what it considers maximal odds.
3) The hover text uses a totally different routine to the AI to evaluate probability. BOTH are approximations of the actual combat code (because the combat code runs several rounds and is expensive, so you cannot afford to use it for odds calculations or turns would run way slower). I'm not totally sure which is the more accurate - they differ significantly when the attacker and defender have very different first-strike counts, which is true in your case (1-5 vs 0)
4) The routine that generates the displayed (hover text) odds is used ONLY for that, and for the evaluation of xp received from a win. It is NOT used for the actual combat at all.
As such I think this is a display issue mostly (really all 3 units have great odds), and slightly a rounding issue (it doesn't choose 99.5% over 99%).
However, if you ever see a case where this happens when it i clear that the chosen unit does NIOT have great odds, it might be indicative of a more serious bug and I'd like to see any such save. For the save you supplied I tried attacks against a variety of animals, and against Neanderthals - both were in the 99% category above.
I also tried it against an archer, which shouldn't be such a push-over, and a longbowman. To my surprise they also came out at 99% odds in favour of the clubman!!!
I have pinned this down to bugs in the withdrawal evaluation in the combat mod, but I don't yet have a fix. However, the AI evaluated odds (which are used to select the attacker and are used by the AI to plan all its moves!) at least (and possibly the displayed odds - I'm not sure yet) are grossly miscalculated if any withdrawal chance is in effect (which i is here due to the 50% withdraw your Great General is giving).
I'll get a fix made for this over the weekend - should be pushed to SVN on Sunday at latest. That won't necessarily correct the display-vs actual mismatch when first strikes differences are significant, but that's a lesser issue. I may try to fix that also - we'll see...
