Can someone explain how this is possible?

Sometimes lag (or a more intentional method) can allow you to take a mon beyond the level you would otherwise be able to, when powering it up.

This Rex was probably leveled up when the multi-tap bug was still available

I'd expect a good portion of players that have 100% Tyranitars to be aware of ways to power up to a +1 level. For example, there used to be a well known method of holding 2 fingers up on the power up button to power up 2 levels at once, skipping over the usual max of 32.5 to level 33.

1:
The multi-tap bug.
2:
Server lagged and clicked many times on the power up button.
3:
Hack the game using 3rd-party apps, against the TOS.
4:
Using multiple devices, also against the TOS.