brmpe Play Store isn't just the storefront, Play Store includes Play Asset Delivery and Play Feature Delivery systems too.
Our compatibility layer includes full support for the Play Store. Play Store services are fully available including in-app purchases, Play Asset Delivery, Play Feature Delivery and app / content license checks.
Source: https://grapheneos.org/usage#sandboxed-google-play
The store is also capable of unattended app updates which could also count for the data usage.