Does Google Play Store change apks with time
When uploading an APK file to PlayStore Google add another signature to the APK file. 

This "Google PlayStore Signature" (also known as "frosting") allows to identify if an APK file has been retrieved from Google Play Store.

Based on the APK signature scheme v2 an APK file can have multiple signatures. 

You can't add add every signature in the signed content the signatures are ignored when verifying the APK (or in detail the complete APK signing block is ignored).
Therefore adding the frosting signature does not invalidate any signatures you have created.

This may be the reason that a APK file retrieved from PlayStore has a different check sum that the one you uploaded.