Why does my outlet tester show that my new, ungrounded outlets are grounded?
I DID ground the lights/outlets to the metal boxes which they are attached to.


Are your wires in metal conduit? If so, you have grounded circuits and the tester is correct.

In any case, on new installations everything should be grounded. Old stuff can get away without connecting a proper ground but not new. Ground for new circuits has to make its way all the way back to the panel.

In addition, receptacles in a garage normally need to be GFCI protected either at the receptacle or the breaker. It is possible to install GFCI without an actual ground, but (a) that is not ideal and (b) that is normally only to retrofit some protection to receptacles that have no ground available. But the ground wire and GFCI perform different functions so having both where required - that always means ground, but GFCI only in certain locations - is the gold standard.