[Nerroth]

When it comes to the Fuyuki Grail system,

Spoiler for Fuyuki Grail:

it had originally only taken 'heroes' until the Einzbern hacked into it in the run-up to the Third Heaven's Feel, so thet they could summon Avenger; after the corruption that then started to settle in, less pleasant Servant options like Gilles de Rais unfortunately became possible.

However, in this case,

Spoiler for Extra Grail:

Apparently the records used in Extra are not from the Earth-based Throne of Heroes, but in the data banks of some Veda-like quantum computer on the moon.

Actually, I wouldn't be surprised if it turned out the moon-puter had some degree of degradation or fragmentation over time; causing certain files to be modified or corrupted over time.

Maybe that could explain certain cases of gender-bending?