I have found out this is how sql servers internal functions work.
This is one of very few places that I have found books online to be wrong.
Books online can be infurating in that even a search for the exact term required will return 50 articles above the one you actually need!
SQL-92 niladic function Value returned
CURRENT_TIMESTAMP Current date and time.
CURRENT_USER Name of user performing insert.
SESSION_USER Name of user performing insert.
SYSTEM_USER Name of user performing insert.
USER Name of user performing insert.
Books online is wrong here