The ID/numbering of ALM's entities is set by incrementing integers to the next available one (e.g. for requirements).  If you have a new project, all entities start numbering at 1.  If you were to import 100 Tests, for instance, it would use Test ID's 1-100.  If you deleted all 100 Tests, then interactively created a test, it would be Test ID = 101.  In effect, you "burned up" 100 id's.

As for having ALM do some different numbering scheme, that would be up to you - done in a User Defined Field, but ALM needs the internally assigned ID to keep track of the various entities.