You can memorize information without understanding what it means.

Retention and understanding are separate. You can hold a definition, repeat an explanation, and pass a test without any sense of why the thing works or when it applies, and the gap only shows when the situation differs slightly from the one you memorised.

A quick check is to try explaining something to someone who knows nothing about it, in your own words, without the phrases you learned it in. Where you stall is where the understanding is missing. That is a more useful diagnostic than any amount of rereading, and it points precisely at what to work on.