I can confirm this. My scroll wheel is very easy to move one increment at a time, and repeatedly moving down one then up one increment results in text moving upwards about 4 lines per cycle. When testing on this site: http://www.javascriptkit.com/javatutors/onmousewheel.shtml I can confirm that scrolling up and down provide a delta of 1200 and -1200 respectively, the way it's being handled on your app must not be even.