I playing vanilla Mount & Blade.

So I'm running around with 57 troops. I see an enemy party with 64. I think, "Hey that's close enough, I can take em!"

Then when it gives me the pre-battle dialog, it tells me I have 57 versus their 120! Where did all the extra enemies come from!?

I lost 10 guys running from that battle, but that's not important. Has anyone else encountered a similar situation? Is this a bug or a game mechanic I don't know about yet?

Do you remember if there were any other parties around when you engaged the enemy? Specifically, if there were other troops nearby who were allied with the ones you attacked (or who attacked you, whichever), they might have made it to the battlefield after you, but before the game started the actual battle. Either that, or there were just two or more enemy war parties traveling so close together that you couldn't see one of them, but if you had to chase them down to fight them, that's less likely, as you probably would've noticed that there were multiple parties at some point during the pursuit. That usually only happens to me when I encounter troops besieging someplace, when they're more likely to be clustered together a bit.

A good rule of thumb, I've found, is that, if an enemy party is confident enough to chase you, you should be wary, as that might mean they can beat you; then again, not everyone who runs from you is going to be a pushover, necessarily. =)

HunchBluntley is most likely right. Usually, if you approch a war party and there are another parties nearby, in the bottom-left corner of the screen, sentences "XYZ's Party has joined the battle on yours/enemy side" (or something like that) will appear, one for each war party nearby (willing to take a side in the upcoming battle).

A few things can account for this.

1-Your soldiers maybe wounded and cannot fight and your numbers are lower than the party number shows.

2-nearby enemies have joined in to kick you ass.

3-Parties from a building like a castle can jump out to fight you sometimes when you walk near. Sucks but it happens.