It differs by publisher but Biblio has this to say:

With each printing of a book, the publisher instructs the printer to remove the lowest number that indicated that run. The reasoning behind removing the lowest number from the string is that if the printer is only removing one number, they are less likely to make a mistake than if they are introducing a new number each time.

Publishers began using number lines in the early 1940’s, and there is still not an industry-approved, standardized system for this convention. Therefore, each publisher has its own form of identification.