I seem to have grant numbers that aren't used. How does that happen?

For purposes of this explanation, pretend that the missing grant number is grant #1. When you click on the new application button, a grant number is assigned. Assume #1.  The next number in the sequence is identified in a table as the number to use for the next new application operation. This would be #2.  So if someone else in the office creates a new application,  #2 is assigned and #3 will be next.  Person one decides that they don't want to save the application and they hit the red cancel button on #1.  #3 is still the next number to be assigned.  #1 will never be available again.

The other scenario is that you deleted #1 after saving.  Because you saved #1, #2 was the next number.  Deleting #1 doesn't restore #1 as the next number.

So to summarize: If you create #1 and cancel the operation before saving, #1 is still available for the next new application.  If you save #1, #2 will be next regardless of what you do with #1.  If you create #1, but someone else creates #2, even if you cancel #1, it wouldn't be available again.