How the updates of some apps in the Play Store start from ~50%? For example, when I updated Whatsapp today, the download started from 13.1MB and the app size was 23.4MB. I had disabled all auto updates. I have also noticed this in some other app updates.
Google Play provides delta updates, which allow you to only download the data needed to patch your existing files to the latest version.  This is done at a binary level, so you will typically need to download more data than has changed, but still less than the entire app.