I assume it's a running success rate. It's making a (hidden) prediction about what key you'll press, and it's displaying the number of times it was correct ove

I assume it's a running success rate. It's making a (hidden) prediction about what key you'll press, and it's displaying the number of times it was correct over all your key presses. So, the more you've pressed the keys, the less each successive press will change it's overall success rate.