Cardplayer odds calculator
I plugged in a few hands and also had a list of playable starting hands. I was really surprised to see A3o (53.9%) came out a favorite to KQs (46.1%). AXo is never even listed as a playable starting hand. Is there an easy explanation for this?

Yes.

If you play it you will lose a bundle to AK, AQ, AJ, AT etc every time an Ace flops.

If you manage to only play it against KQ then you will win in the long run, but that is like saying 73o is favourite against 62o and should therefore be played.

The A3 is a favorite over KQ because it's already ahead and doens't need to improve to win.

Be careful with those odds calculators for a couple of reasons:

1. That we know the odds assumes we know what our opponent is playing, and unless you're cheating or he's telling you, we don't.

2. They assume that the hands are played to a showdown. How often will you stay in a hand with 44 against a flop like Q J 7, even when your opponent has AK? While you have the best hand right now, it's very difficult to know that, and you opponent has 10 outs (22% to hit on the turn) to a more or less unbeatable hand.

I think when a lot of players put in small pairs against AK and find out they're actually a statistical favorite PF, they then tend to overplay these hands.

very good answers, I think I have been overplaying certain hands :P

KQ does better vs a random hand than Ax.