Sparse numbering of Lead IDs

Following the approach described in http://developers.marketo.com/blog/get-all-leads-from-the-marketo-rest-api/  to retrieve all Lead records, I noticed that in the system I'm working on I have many "holes" in the Lead IDs; so, for example, I ask for Leads 3100100, 3100101, ..., 3100199, and I get back only lead 3100187. In which situations would that happen? Are all the "missing" Lead records deleted ones?

Thanks.