How do successful undergraduate and PhD students differ?
I'm not a big believer in "types" of people. But yes, some succeed in an undergraduate program but don't in a doctoral program. The difference, I think, is largely determined by the nature of the programs, not the people. 

As an undergraduate you are mostly exploring what is already known and the connections that are already known between them. Too often the method of evaluation is poor. Poorly designed multiple choice questions, for example, test memory primarily. Memory is a useful skill, but more is needed to complete a PhD. An undergraduate degree, even when focused on a particular field, as in UK, is still pretty broad. 

A doctoral program, on the other hand, is an exploration of the unknown. You can't plan for success in research, only work effectively so that you can find it eventually. But it is much more frustrating for many people than is the exploration of the known. You are trying, among other things, to make connections that others have not yet made. It is a different kind of thing. The boundary between the two isn't smooth. 

So, the undergraduate degree is only a partial preparation for what needs to happen in a doctoral program. Being successful in the one doesn't guarantee success in the other. It gives a basis on which to work, but that is all. Some people learn, too late, that they just don't like researching the unknown, even though they love what they did as undergraduates. 

In mathematics, for example, if you are studying things already known and come to a hard part, you can find someone, or a book, say, to explain it to you. You can find exercises to deepen your knowledge. But as a mathematician researching some area, there are no such resources. You are a creator, not a consumer. There is no guarantee that you will ever succeed in answering some particular question. 

So, I attribute the effect you see more to the different nature of the two levels of learning than to the "types" of students who succeed or fail. 



However, as an afterthought, I think that most people will start out in their education finding things fairly easy to do. The things are fairly simple, of course. But usually people reach a point where it isn't easy anymore and they have to work to continue. I think that the people who don't reach this point early in an undergraduate program, or at least by its completion, will have a hard time in a doctoral program. If you've never had to work hard to advance, research can be a shocking awakening. I was lucky enough to learn my limits in secondary school and so learned to work hard at learning. Others, who far surpassed me early on, didn't advance as far. I doubt that this is a universal, but think it is pretty common.