Curious too. I've seen that happen on some Amazon apps, but usually they won't install over the Amazon one. 75% of my free Amazom apps have higher versions at the Playstore. Seems Amazon gives you the free app, but developers then don't update the Amazon version, so you have to go buy the upgrade version at Google. Quite an Amazon scam.... but doesn't bother me much since usually the app doesn't change much.
One that's really squirrely is "Sketchbook Mobile". Seems the Play store updated the Amazon app I have with that name with "Sketchbook Pro for Tablets", and there are 4 versions at the Playstore:
"Sketchbook Pro for Tablets" << this one has the Amazon icon and is one that it updated to.
"Sketchbook Mobile Express"
"Sketchbook Express for Tablets"
They all seem to do about the same thing...