Incorrect display of checkboxes

Incorrect display of checkboxes

Hello,
why can it be that some checkboxes are displayed incorrectly?

Here is an example: http://loefe.de/jq/

I have tried the current v1.12.1 as well as the v1.11.4. In both cases, the lower checkboxes are displayed incorrectly.  Tested with Seamonkey and Opera Browser.

It seems as the combination of tab with control group and checkbox is causing problems.

I hope one of you can help me.