What a person knows or does not know when you dive with them is only somewhat correlated with what they are taught. I teach in a university and I have 100s of examples of where I know they have been taught X but have not used it for a couple years and no longer remember X. Any instructor who assumes that every student they teach will remember everything two years later, unless the student keeps learning, is naive.
On a personal note in my senior year in college I was taking an advanced math course and saw the definition of a limit. I thought it was a new idea and learned it (and still know it). A little later I was looking at my freshman calculus book and found the definition of limit: underlined in black, highlighted in yellow, blue box around it, and three red stars in the margin (no exaggeration) Most highlighted thing in the text. Apparently I had memorized it my Freshman year. But not using it, had filed away and forgotten.
On a personal note in my senior year in college I was taking an advanced math course and saw the definition of a limit. I thought it was a new idea and learned it (and still know it). A little later I was looking at my freshman calculus book and found the definition of limit: underlined in black, highlighted in yellow, blue box around it, and three red stars in the margin (no exaggeration) Most highlighted thing in the text. Apparently I had memorized it my Freshman year. But not using it, had filed away and forgotten.