I've been nearsighted since I was a kid. My guess is that myopia isn't so much a disease, but rather your eyes becoming specialized in what you use them for. Lots of close range and dim environments (like indoor lighting) = eyes that are good at seeing in that environment.

One of my eyes is significantly more myopic. One eye was about -4.5, the other -6.5. Despite that I will frequently use my phone without glasses. This made me realize that the "worse" eye can actually focus on what I'm looking at a slightly closer distance than my "better" eye.