Practice makes perfect:
Evolving Attacks are very easy to justify seeing that, in real life, repeated use of a skill causes one to naturally get better at it.In the case of physical activities, practicing the same movements over and over again is the key to getting them ingrained in one's muscle memory to the point that said skill becomes second nature. A dancing routine will flow through your body, one step after another, so you can work on doing it more gracefully, which in turn will improve the quality of your performance. Likewise, a martial art kata will build the muscles it uses so each strike hits stronger and is more efficient.
Cognitive abilities run along similar lines but often aren't as straightforward. Reading faster is great and all but does very little if comprehension is not upped as well. Meanwhile, things like memory are kept by long-term strategies such as periodic recalling instead of the very short-term, so-called photographic memory.
If a creator likens whatever magical or Phlebotinum-related ability to mundane skills, then it's well within the Willing Suspension of Disbelief to think it evolves the more the characters use it. Often, the difference is that the former eventually hit a peak while the latter can rocket way up their initial caps if the plot so demands.
