>>47247576Hitmontop isn't just part of the only evo line to connect two previously unrelated pokemon and the only evo line besides Eevee to have more than two split evos, it's also one of the very few pokemon to lose a move from its learnset. Hitmontop could learn Drill Run from the B2W2 move tutor, but not the ORAS and USUM ones. This was only recently rectified in gen 8 where it learns Drill Run by TR87.
Another pokemon this happened to is Weepinbell and Victreebel, who could only learn Bug Bite from the B2W2 move tutor and not the ORAS or USUM one. Gen 8 wouldn't fix this not only because they weren't dexited, but also because there's no TM, TR, or Move Tutor for Bug Bite. This may change in the BDSP or Legends Arceus since they're in the sinnoh pokedex.
This is even weirder when you compare to Elgyem and Beheeyem learning Steel Wing because it has the same TM number in gen 6 as Ally Switch in gen 5(TM 51). This was clearly erroneous, but the two are still allowed to learn Steel Wing up to and including Gen 8 without transfer. For some reason, they remembered to preserve this error but not the two prior move tutor moves.