Ingenuity is a unique talent of a scientist. Unlike their other skills, which will improve with practice, a young scientist may already be quite ingenious. While ingenuity can be improved, it's rarely from practice, but often from Eureka moments, and more difficult for older researchers.
Some Points of Interest can also have an effect on ingenuity leveling rate.

Ingenuity determines the chance of coming up with an exceptional contribution to a research project. That can mean simply a better result, but also a hybrid. When several rare ideas come together in a single project, the result can be quite impressive.