When you put a content in a colspan which is larger than the width size of their children then the darwing of the grid fails. You can see this in the following example:
http://jsfiddle.net/omhardb/Lswzvfen/. If you see the witdh of the content columns don't match with the width of the header.
Please let me know if i need to put a patch or if there is a solution for this.
Thanks,
PD: Sorry for my bad english