My opponent had a gabrien archon which was a 6/6 from playing two cards that cost 7 or more, it was then buffed to an 8/8 with tiki caretaker to an 8/8. My opponent then played Apex predator targeting the 8/8 but the predator became a 13/13. I’m pretty sure 8+3 =/= 13, however.
Did he have any creatures buffing others in hand? Did he use a buff draw? There are plenty of green and blue cards that could’ve caused that.
Based on the math, 8+3 gets the Apex to 11 so its missing 2/2 to get to 13. My guess would be a Triton Trainer was played earlier in the game since your opponent was playing Blue and Apex has Jump so it would get the buff.
The in-hand buffs as Galileus suggested could work too, +2/2 would be Court Jester or 2 Spirit of Rebirth triggers.
But my bet would go to a Trainer buff.