Where do these version numbers come from “module-name-1.0-alpha1+13-dev” with plus?

Question

I've started to work in an existing drupal development project and wanted to do some module updates and found modules with versions like this:

• Universally Unique IDentifier 7.x-1.0-alpha3+47-dev
• Taxonomy CSV import/export 7.x-5.10+17-dev

I've never seen this before. According to drupal version number guidelines this is not wanted.

Now, our head developer says they never modified any contributed modules and only took them from drupal.org ... Where do that version numbers come from?

Did I miss something?