Time Machine backing up more data than I can physically have
I'd say that the Info Window on your Hard Drive that you posted in the comments is completely correct. It uses the new base 10 system for calculating your Hard Drive space. 

I guess Time Machine is misreporting the amount of data backed up. This might be due to either Time Machine still using the old base 2 method for calculating HD space, or by Time Machine getting confused by the use of symlinks and adding up the size of some files multiple times.

It also could be that somehow the backed up data is bigger than the original data. This could be due to metadata, error recovery, or other factors. But I don't think this is the case, because the difference that you report is quite big.