When you download your data into the iPad Hub, it requires a completed table chart with your groups assigned.  Any table not assigned a group will automatically have a group called Overflow created in order to fill that table.

Before you download your data, be sure to create your table chart and assign your groups to it in order to prevent the Overflow groups from being created. 

You can delete overflow group by clicking into the group and clicking the trashcan icon in the upper right corner of the group.