@nyn: aren't you lucky. You can make the toolkit do something that I can't.
There is nothing to check. The 10-15 pixel is a 'little bit of space' that appears between the boxes and the scroll bar. Clearly you do not have any space. The reason seems to be the length of the column names - you have a long one. When I made one of mine very long, I see the same problem.
The change I made does not fix your problem. The group box gets bigger, but the scroll bar remains over the controls.
More thought is required.