Things turning from something they are into something they're not happens all the time in the Nasuverse. It's really not an uncommon thing - people and legends change based on humanity's perception of them. There's a multitude of Author Servants who are competent Casters, but they're literally just well-known Authors in our world (Hand Christian Anderson, William Shakespeare, etc). Semiramis has a NP that was never her own because people attributed it to her legend after her death. Hell, there's even Servants whom are able to be summoned even though they never really existed - The Phantom of the Opera is a Servant in Grand Order.
I mean, it's really not that strange that they were made Servants. Leonardo da Vinci was turned into the
"image of the Mona Lisa" because it's one of his most well-known pieces. It happens to a lot of Servants.