Delusions and misbeliefs

Beliefs may be true or false, and grounded or ungrounded. McKay & Dennett (M&D) treat these properties of belief as independent. What, then, do they mean by misbelief? They state that misbeliefs are simply false beliefs. So would they consider a very well-grounded belief that is false a misbelief? And why can't beliefs that are very poorly grounded be considered delusions, even when they are true?