If you look at the documentation for something like "create sequence", databases often mention that the database will reserve blocks of numbers so that they can

If you look at the documentation for something like "create sequence", databases often mention that the database will reserve blocks of numbers so that they can hand out values from memory (and also without coordinating with peer databases). There's normally a way to turn that caching off. But you may still need to check what the behavior is during a transaction, as you could have two requests trying to add a bunch of rows at the same time.