Google play store users count vs my own users count

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http://stackoverflow.com – My active users count on google play store is showing a much bigger number than my own users count. over time, over app versions. the count is bigger by ~25% (10s of thousands of users). My count is based on a ping that send every time user is install or using the application. if there's no internet at the moment it saves the ping to a time that internet will be available. there's no use to my app without internet, so I guess the users is not blocking internet to my application. Any idea why? (HowTos)