Header and Row misalignment whenever scrolled horizontally

Hi, Loving the work!

Bumped into another issue. After showing grouped columns, my header gets misaligned with the data rows whenever I scroll.

Screenshot: http://i.imgur.com/Yt91Hio.png

Sample: http://live.datatables.net/ludiqoyi/4/

It happens when the x-scrollbar shows. When I scroll data rows, header stays fixed. Am I doing something wrong here?

Thank you!