You may have noticed that some Scott catalog numbers are not used. For example, Scott #1101, 1102, and 1103 are not used. Why?
Originally, Scott #1101 is now #1042 (8¢ Liberty), #1102 is now #1046 (15¢ Jay), and #1103 is now #1048 (25¢ Revere). When those stamps were issued, the Liberty series was supposed to be finished. The catalog editors didn't leave gaps in the catalog numbers to include these issues. Originally, they were given these catalog numbers. By the time the 1960 US Specialized catalog was published, the catalog editors had re-assigned some numbers to make the Liberty series sequence complete. They never did re-issue those 3 missing numbers.
Likewise, Scott #779, #780, and #781 are not used. When the #778 souvenir sheet appeared, each of the 4 designs was given a separate Scott number. Remember, this was the time when the Farley issues were released and issues like the National Parks had separate catalog numbers for both the perforate and imperforate varieties. The designs used for the #778 souvenir sheet also had perforated copies. It was a natural choice to give the 4 stamps in the #778 sheet a separate catalog number. At a later date, the Scott catalog changed the listing to only #778.
The catalog editors try to reserve a block of catalog numbers for definitive issues which appear over several years. It's hard to guess exactly how many catalog numbers will be needed in advance. That's why you'll find catalog numbers like #1031A where the "A" is added to squeeze in an item. Or sometimes a few catalog numbers are not used