Yeah. That's why you really have to get a feel for what series are good if you ever want to make any money.
For example, never bet DBZ. Their A.I. is complete crap 99% of the time, but they put out huge damage when they attack. Sometimes that can cause a serious upset. The only ones that I've seen that are actually good are Perfect Cell, and Dr. Gero.
Guilty Gear and Jojo all so used to be in the "never bet" category, but recently he's downloaded new models with better A.I.
If one of the fighters is a little girl she's probably a safe bet. As are any characters from Melty Blood, and anyone that looks like they could be in Marvel vs. Capcom. It has a very distinctive sprite style that's easy to spot once you know it.
If someones A.I. breaks and they start attacking before the betting period is over, they're usually going to win.
Hey son, wanna' learn how ta' make witch balls?