Because you don't stand over your bike when you ride it and you don't need to stand over it when you dismount. I'm also short, and have no standover clearance on over 1/2 of my bikes. No problem, because when I stop I dismount from the saddle, put my left foot down and keep my right foot on the pedal, such that the bike is tipped on its side and I easily clear the top tube. What matters is your reach from the saddle to the handlebar, which is a combination of top tube length and stem length. Smaller size frames have shorter top tubes, and you probably have a short torso so you don't want a bike with a long top tube. If you get a larger frame size, you will need a very short stem to compensate for it. Anyway, based on the info you've provided, I'd recommend the 44cm size as the best choice.