Why some consecutive numbers are not assigned for the identification of records

Sometimes the system assigns consecutively the record identification number, but for several objects leads, opportunities, accounts, the application sometimes omits a number or a consecutive number, for example, I created several leads almost at the same time but the application for some reason for the lead number field did not assign the number 12422. This number does not exist in the application nor does it belong to any previous lead. I send image for reference:

I want to understand what is the reason for consecutively assigning a number but sometimes it omits some numbers or values.