All lenses have some innate distortion. In good lenses it can be hard to see, sometimes, but its there. Manufacturers use tools (maybe like this one?) to analyze their lenses during design as well as after production for consumer models and create corrective models.

So when I take RAW files from my canon DSLR and load them through a compatible raw processing software, it knows the lens and camera used to take the picture and it will correct those distortions, and sometimes deal with chromatic aberation etc.