Google play vs. the Nexus itself's apps

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http://forums.androidcentral.com – Many times I will go to Google Play, My Applications, and notice that several apps I own have updates. When I send those updates to the Nexus 7, they frequently get the error "application already exists" once they arrive. My question is, how does the Nexus 7's app versions get "out of sync" with the Google Play store? I've watched carefully and the store lags the actual automatic updates by several days in some cases. Google Play offers to update them for me when I know they've recently been updated automatically. What gives? (HowTos)