Form submits differently using 'enter' and submit button.

Now this is totally weird...

I have a registration page created using 'profile' module. So besides standard 'user name, pass, etc' fields there are around two dozen fields from one of the profiles user can have. Let's say 'competitor' profile.

Only yesterday I have noticed, that if you submit form hitting enter, you get additional fieldset and legend tag - with the name of the profile ('competitor') which wraps all the profile fields. When submitting by a button click everything works as expected - form looks like before submitting, only with validation messages and the like.

If I submit first with enter and then with button - fieldset is there again. Only loading page again makes it disappear.

Any idea what's it all about? I tried to compare http queries, but they seem to be more or less the same...