Really it could be anything, from oil weight to actually damaging something in the shock, to merely a seal issue (allowing easier internal air movement).
If you did not change the seals but it was dry when you opened it up and you opened it because you heard rubbery noises when you compressed it, then i'd be putting my money on it needing some internal seals changed.
Alternatively your shock might be totally FINE! You might have just messed up your shock pump gauge and be seeing the wrong pressure. Borrow another shock pump/guage etc to check.
The good news is you probably haven't ruined it, in the sense you need another shock, but I wouldn't go riding it until you figure out what's wrong.
The bad news is that we can't REALLY help you without actually having you photograph the shock as you take it apart and put it together again, in extreme detail.
It's unlikely that nothing went wrong (unless it's just your gauge showing the wrong pressure all of a sudden); you would still get similar travel from the unlubed shock, it just wouldn't be as smooth.