Whatsapp for android is downloaded in two main ways. firstly via Goople Play store and secondly directly on the whatsapp website. The website download is actually a beta app. take note, most of the time the version number on the web download is ahead of the play store version, the silent feature in the web download / beta app is that it is self updating. took me a while to notice this. i would recieve a notification on launching the app that an update was available and ready for installation. you can imagine my anger at realising where my data was going to.
On the hand, the google play version is manually updated. the reason for this is Google doesnt allow any play store apps to self update without going via google play. these are goodgle's regulations. my advice, stick to google play apps.