there are a lot of crawlers out there on the net that might trigger autodownloads.
I have had bad traffic experiences where I bought from dodgy second tier se and then got 100s of downloads but no rmps, hence the warning.
I wouldnt put it past them – or their search engine results resellers – to use autocrawlers to effect downloads to skew the stats so conversions look better
Also some crawlers are just plain dumb and follow every link…