I think that it's possible to have a delusion that is technically possible. So, for example, if you believe with 100% certainty that you are a genius, then you are delusional (assuming that you're not a genius). If, however, you just think you are a genius sometimes, but don't really believe it (maybe you just feel smarter than others) than it's grandiosity.

Once while manic I believed that I was the "smartest person in the world". I really thought that I was going to do something incredible. I don't know whether it was a delusion or just grandiosity, because I can't remember how strongly I believed it.