Some were due to multiple events in one year and some I believe were lost during challenge matches.
In the case of Alfredo DeOro, whenever he won a pocket billiard championship, they would take away his billiard title and then the next time he won a billiards title they stripped him of his pocket billiards title, in reality for a 20 year span DeOro should have held the titles to both.
Almost no one beat him , they were just the recipients of the vacated title.
I may be wrong but I believe that for many years, particularly during the 20's, 30's and 40's, there was one World Championship Tournament held during a fiscal year. Although, the Champion could be challenged by either the Championship runner-up or the number one ranked player in the time between tournaments.
I like the idea of challenge matches. Its perfect for the TAR and stream era. Its like boxing. Plus not everybody goes to the WPC so it just adds to the hype if they held those matches today