Why do I get "Successfully Downloaded" when not all records make it in?

I load leads, get the message that the load was a success, only to find via Data Gaps reporting that not all records are accounted for. I'm new to Marketo and would really appreciate guidance on what I am missing in this process.

I resolved the problem by setting up Data Gaps that look at Unsubscribes, too. For example, the Data Gap "Facility Type is Populated for Subscribes" wouldn't account for all records, but if I look at this count in combination with the Data Gap "Facility type is Populated for Unsubcribes", I account for all records expected.