The numbering of Popes John does not occur in strict numerical order. These discrepancies in regnal numbers are due in part to a now discounted belief in another Pope John between John XIV and John XV, and the antipapacy of John XVI.
John XX
Confusion in numbering Popes John results from an error in the textual transmission of the entry on John XIV (983/984) in the Liber Pontificalis. This entry originally specified not only the duration of his pontificate ("VIII mens." = eight months), but also the duration of his ensuing imprisonment by Antipope Boniface VII, "per IV menses" ("for four months"). In the 11th century, some time after the pontificate of John XIX, this entry on John XIV was misread to be referring to two different popes John, the first reigning for eight months and directly succeeded by another John reigning for four months:
Iohannes m. VIII ("John, eight months") Iohannes m. IV ("John, four months")In distinguishing these two Johns, the second one came to be numbered as "Iohannes XIV. bis" ("John XIV the second") and it was confused with a historic character, the cardinal deacon John son of Robert, who opposed Boniface VII after John XIV's death. Given the fact that the following Popes John, from John XV (985–996) until John XIX (1024–1034), seemed to have neglected the existence of John XIV "bis", Pedro Julião "corrected" this error by taking the name John XXI rather than John XX.