Why app updates download entire app instead of only "updates"? I find it rather strange that for updating an app in Android (or even iOS) the entire app is downloaded instead of only the actual "Updates".

The reason I find this strange is because in Windows while updating an app, only the update files are downloaded -- not the entire app -- which is more practical as it saves both time and data usage.

This would seem trivial to most, but when you need to download a huge app just for a tiny feature addition/bug fix it becomes a real PITA.

Does anyone have an explanation as to why this model was implemented?
As far as I know it does download just the updates (referred to as the "delta") on Jellybean 4.1 and above. Obviously the initial install is the full app but updates only download the data that has changed from the current installed version (i.e. the "delta" of it).
You won't see anything different when updating most app, it will still look like it's downloading the full app but on some larger apps you will see if 'jump' parts of the download which indicates that it is just getting the delta.

Engadget and AndroidCentral both have articles indicating that the Play store does provide delta updates