Why does Play Store want to update my Amazon app?

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http://androidforums.com – I bought the ad-free version of Angry Birds - Star Wars a while back from Amazon (v. 1.4), since Play Store didn't offer it. The Store said an update to v. 1.5 was available, so I installed it since Amazon shows no similar update. None of my progression was wiped, but I didn't see the extra level it promised so I reinstalled the Amazon version. I'm just wondering, since Amazon recognized that v. 1.5 want theirs, why Play Store didn't do likewise. (General)