Fire Red AI mystery [likely solved]

This mystery concerns the Heracross in the Fire Red Round 2 Champion fight. Its moveset is Megahorn, Earthquake, Rock Tomb, and Counter.

When you outspeed, Heracross' behavior against various Pokemon is as follows:
ZAPDOS: ~72% Rock Tomb, ~23% Counter, 5% Megahorn
CHARIZARD: ~75% Rock Tomb, ~25% Counter
BLASTOISE: ~47% Rock Tomb, ~47% Megahorn, 6% Counter

When you are slower, the behavior changes:
ZAPDOS: ~90% Counter, ~10% Megahorn
CHARIZARD: 100% Counter
BLASTOISE: ~90% Megahorn, ~10% Counter

Heracross uses Rock Tomb more often on Zapdos and Charizard because Rock Tomb is usually the most damaging move (against Zapdos, Megahorn occasionally rolls higher than Rock Tomb). It uses Counter more often on Zapdos and Charizard because they have a physical type (Flying).

When you are slower, Rock Tomb is dismissed. However, before that happens, the game decides its most damaging move and dismisses all others. This explains why Megahorn is not possible on Charizard, and very unlikely on Zapdos. A step-by-step breakdown of the AI logic against Charizard shows this:

CHARIZARD
STEP 1: Choose most damaging move (always Rock Tomb)
STEP 2: Dismiss moves with Speed-lowering effects (always dismiss Rock Tomb)
STEP 3: Choose between most damaging move and all other moves that have effects (this is between Counter and Rock Tomb since Megahorn and Earthquake do not have effects, and since Rock Tomb was dismissed, Counter is always chosen)