It's a bit bananas, but probably just because he could. He also wrote his personal notes in "mirror writing":

> The notes on Leonardo da Vinci's famous Vitruvian Man image are in mirror writing. Leonardo da Vinci wrote most of his personal notes in mirror writing, only using standard writing if he intended his texts to be read by others

https://en.wikipedia.org/wiki/Mirror_writing

Da Vinci was showing off.

just rotate your monitor