Thanks for the replies.
First, your simplified code does look better. I'm self-taught at this and it doesn't come easy.
Second, the radio buttons really do make sense. I think when I coded it a few years ago I just wanted to use text boxes for all input for consistency. Actually, I think I'll just make it a drop down menu.
That will certainly eliminate the problem, though I will remain curious how it could happen so irregularly.
FYI my webpage is http://www.caloriesperhour.com/index_burn.html