Now that I have more data it is a little odd.
I have 1 (and only one) that does not show a source. If I add up the hits from the referrers page it is exactly one less than the downloads counter. At first I thought this was because the referrers were not tracking downloads directly from my site. However I now see that is not the case.
At this point, that one download is insignificant unless it starts repeating. I will know more in a few days.
Are you planning on adding the time filter to the referrers tab?
Being able to filter it like the Referrers Pepper (repeat tab) would make this perfect for mel.