App Slicing Thinning: Pre determine actual true size & space taken by App install or update on iOS Device?
I can see multiple reasons for that happening:

A) Compression

Apps are downloaded in compressed form from the app store. It is similar to a Zip-file. When the app is installed on the phone, it is uncompressed by the app installer software on the phone.

B) Differential updates

If you already have the app installed, and are merely downloading an update for the app - you could be downloading a differential instead of the full app. For newer iOS versions, the App Store will calculate the difference between what you already have and the new version of the app, enabling you to download less. However it will still take up the full amount of space on the disk after the upgrade has taken place.

C) App downloads after first use

When you use an app for the first time, many apps will download extra data that is stored with the app on your phone. This could account for differences in app size as listed by the App Store, compared to the amount of space the app takes on your phone in practice.