I suppose A2o is slight favourite because if neither hand hits anything then A2o would win - unless the community cards are the best hand ie. 56789 straight, then its a shared pot. You both have the same odds of hitting a pair - but if an Ace hits then A2o becomes a big favourite. I'd much rather be holding A2 in a heads up pre flop showdown - but its still a coin flip situation really.
There are plenty of sites on the internet showing how to calculate odds of hitting cards which describe them much better than I could. Search away - you'll find one.