6.0 marshmallow - Google Play store asks to uninstall apps on sd card to free up space for new installations

My phone internal storage has very less free space (about 450 MB), which is why I cannot install any new app, or update existing apps.

When I go to play store and click on any app to update, it asks me to uninstall apps to free space. However, the list also has apps that are completely on the sd card. When I click on the check box adjacent to an app that is on sd card, play store says that the required space has been freed.

But how is this possible? Apps on sd card should not take any space on internal storage other than cache. On the other hand, updates are always installed on internal storage. So, how can an app on SD card free up space on internal storage?

For example, consider the following app which is on SD card:

I want to update an app, which requires 46MB storage space:

When I tick the check box adjacent to it, play store says that space has been created for update to get installed:

How is this possible?

Answer

See App Install Location for what could be the reason

When your application is installed on the external storage:

The .apk file is saved on the external storage, but all private user data, databases, optimized .dex files, and extracted native code are saved on the internal device memory.

(Emphasis Supplied)