The number of leads can be fewer than downloads if you have targeted your leads campaign to a particular geography.   The Analytics section of your SlideShare account has users from other geographies that would be able to download your file but not be able to see the lead form (as the leads campaign is geo targeted.)

For example, if you have geo-targeted your leads campaign to the United States; the leads form would only be visible to users from that region.  Though users from other geographies would be able to download your presentation, the lead form would not be visible to them.  

You can switch off geo-targeting if you would like to collect more leads on your content.