I know I've been bitten by this in the past. In particular, once I select ++ the focus can remain in that section of the form so that interactions with arrow keys, pgup/pgdn, etc, will actually switch the radio selection to a downvote (--) rather than DWIM. To avoid it I always click on the page background to ensure the focus is no longer on the radio buttons.
I should add that other interfaces, such as mouse wheels, do not seem to present this problem. But when you have little control over the computer layout (travelling) then you are at the mercy of whatever interface is available.