I asked a Google Rep about this and here’s the response:
To ensure the Similar Apps section promotes discoverability and shows new or different apps to users, the apps are algorithmically selected and change regularly.
Also, if we always showed the app the user just came from, the number of apps that could be displayed in Similar Apps would sharply decrease, and the feature wouldn’t be as useful to users. Over time, the apps that list your app in their Similar Apps section will change.
This is to keep Similar Apps results fresh for users and to give other apps an opportunity to succeed on Google Play. Because of this, your app will not always be shown in the Similar Apps section of apps that are listed in your app’s Similar Apps section, though it may appear there from time to time.
Due to the constantly changing nature of Similar Apps and Similar Apps algorithms, I’m afraid I’m not able to explain how “similar apps” work