Some 11th century historians believed that there had been a pope named John between antipope Pope Boniface VII and the actual Pope John XV; thus, the series of Pope John XV to Pope John XIX was mistakenly numbered John XVI to XX. These popes have been renumbered as John XV to XIX; however, Pope John XXI to Pope John XXIII were numbered based on the confusion of having twenty Pope Johns before them. Hence, Pope John XX doesn't exist, having been skipped over in the enumeration of popes, and as the enumeration is now firmly established, it is unlikely that any attempt at rectification will be made.
Some have used this discontinuity in numbering as "evidence" that there was a Pope Joan: see myths and legends surrounding the Papacy.