Hi Nick,
I don't have a great answer for why those differ. I don't think anyone on this list is an expert in the inner workings of HOP or Rockstar. The fact that HOP and 1.5xRockstar agree so well for large radii is suggestive of a missing little h somewhere. I assume that both of their virial radius calculations are done with spherical overdensities. It may be worth taking a few cases and using the HaloCatalog to calculate a similar R200. There a way to do that here:
Sorry, that's not a great answer.
Britton