I was comparing the same download stats for two files for a specific date range in the GUI of Piwik and in an exported TSV (I opened the row evolution and exported the stats as TSV). I use Piwik 2.12.1.
They did not match and differed ~25%:
In the GUI the (aggregated) download number was lower, the sum of the downloads over the whole date range in the exported file showed a higher number.
I tried different data ranges but could not find out what the specific error pattern is. For one file the error seemed to occur when the download number crossed the 10k mark but this didn’t apply for the other file.
I could not find any reported bug on this issue and this error just seems to affect stats within specific date ranges (monthly stats seem to be correct) but I would nevertheless estimate it as a seriuos bug.
Could you please take a look into this matter?