Technically, it IS 8GB, but that disk was configured with a "virtual RAID" (notice the use of quotes there...pardon me for not going back and looking up all of the technical jargon again to supply a more accurate answer) to allow for greater reliability and reduce data corruption.
In the end, you're left with about 2.5 GB of usable internal storage. Given the fact that the TB ships with a 32GB SD card, that should be plenty, but HTC/Verizon shouldn't have advertised 8GB regardless. This has been discussed ad nauseum already, so it's really not worth getting into it, but that's the situation.