I've noticed my potential vote switching over when I use a wheel mouse to scroll. The radio button is still 'selected' and the wheel switches between values, rather than moving the page up and down. It takes a click into the body of the page to 'switch' away from the radio button. Could this explain anything?
I've noticed a similar behavior. I don't have a mouse wheel,
but I use Page Up/Down, and it changes my vote. I think it
has to do with the notion on Windoze that controls deserve
to get the keyboard focus. Being an unreconstructed Mac
user, I expect TAB to cycle among the text fields (possibly
to include select lists), but *NEVER* to focus on buttons
of any sort.