If on a multi-player server my guess would be that someone else is taking the other 17. If you are playing single player the only things I can think of is that you ran over a few, put them away and forgot, or it is indeed a bug, or you are using mods that might be causing the issue.

Has this happened more than once with no mods installed on a single player game? If so I would suggest reporting it in the bugs sub forum.