Compare to APKs distrubted in other platforms (Google-Play,etc.)
APKs can be distributed in other places as well, such as:
GooglePlay https://play.google.com/store
Samsung Galaxy Store https://www.samsung.com/global/galaxy/apps/galaxy-store/
Huawei AppGallery. https://huaweimobileservices.com/appgallery/
Amazon: https://www.amazon.com/androidapp
Aptoide https://en.aptoide.com/
more complete list at: https://en.wikipedia.org/wiki/List_of_Android_app_stores
If the files are identical it's signal that it's more likely legit APK (as some stores have testing to detect malware)
If the files are not identical, it will be useful to have diffoscope result between them and what published in fdroid, some apps will have special different version on fdroid due to different build-flavour which not include/use google-play-services, or other closed-source-libraries, etc.
The diffoscope result will be useful to see this changes, and to create guidelines for developers to reduce this diff if possible (so verification will be easier, etc...)