That's right -
It is kind of a hard concept at first, but in InfoPath, the data source and the controls are two separate things. The control is bound to a data source field, but that doesn't mean other controls can't be bound to the same data source field.
So, you could have two repeating groups, and programmatically copy data from one to another. If data changes in the first group, unless you have logic (rules, defaults, code) in place to change the data in the second group, the second group's data will stay as it was when you populated it.
On the other hand - as in my sample, where there are two controls bound to the same group, you'll notice when you add rows to the first table, you see them in the second table as well - that is because new nodes are being added to the repeating group the table is bound to.
Hope that helps clarify a little bit?