Well, there are likely several potential reasons and if very intelligent, boredom is high on the list. Online tests aren't the greatest measure though.

Most likely though is that his memory is not good for certain things. Intelligence and adaptability are different to recall ability. I have a terrible memory for school but, I am relatively adaptive in the moment so even though not intelligent, I can shift my thinking faster than most knowing what I know.

Memory is a resource, not an active tool. it is what can be done with what one has that is important.