opps I forgot there are 2 downloads listings ! 

so more precisly the last 3 days count is intended for the "Yesterday" Top Skinners Author list
regarding the Popular Skins list the "Yesterday" count is referring to the last single day anyway I noticed that the skins appear randomly as each skin will not be in the list for more than 2\3 days but it can reappear after 2\3 day
<< Note: WinCustomize works to ensure its download counts are accurate. Some download sites inflate their download counts either intentionally or unintentionally. On WinCustomize.com, we take measures to try to provide you with accurate download counts. These download counts can be assumed to be fairly close to the actual # of individuals who have downloaded this skin, theme, wallpaper, or icon package.>>
well in my experience I trust it and after all to do a sum is a very basic operation !