Kind of makes you wonder just how often that mix-up happens. Saber told me once she knew of a Heroic Spirit who had to disguise her gender so that she could become a king.

[Hahaha Hakuno if you only knew...]

... But in Saber's case it sort of just seems like the historians were lazy. She outright told me the reason she dressed as a man was because she "liked the outfits", not because she had to hide who she really was to rule.