Well, there's a difference from making them distinct and making them the exact same.

Porom's equipment choice in FF4A is much better than FF4. FF4A allows Porom to use the Assassin's Dagger plus a number of better options for Agility increases.
They both start with almost the same Agility at level 70 (Porom has 33, Rosa has 22) with Rosa having better overall stats.. though Porom having a bit more Will (Porom 75, Rosa 71).
Porom's weaponry can increase Agility, in the form of the Assassin Dagger (+15 Agi), where Rosa's only option to increase Agility via weaponry is the Artemis Bow (+10 Agi). They both have similar armor sets that increase Agi equally there. The Agi is critical for me though...
If you were to use FF4's equipment data, Porom literally has the same equipment set Rosa has (I could be wrong, but I believe this is the case). This factored with Porom having weaker starting level 70 stats by comparison.. it becomes apparent that some equipment (and preferably stats) have to be heavily tweaked.
I hadn't really noticed the level up data to be that significant before.. but maybe I'm underestimating Square when they did them since they are done pretty well and scale correctly until post level 70 (where there's almost no guarentee that Rosa has 99 Will Power via random chance).