Honest question here, why does fizz ult do 1200 damage and then apply a bleed.

Ok here's math.

You said 500 ap then lets go with that. Fizz's small R deals 350 + 0.6 ap, that's and minus 33% when counting mr(takin in 50 mr as it scales with most champions and most will have runes, its the minimum). Which is 435.5

Mid shark is 425 +.8 ap. 825 dmg, after mr 552.75.

Large shark is 500 + 1.2 ap. 1100 dmg before mr, after it's 737.