Because they have different appid. Izzy version is app.organicmaps.web fdroid version is app.organicmaps (you can see this in the url). Google version uses the same appid as fdroid. You can't have two apps installed with the same appid, but this way you can have both installed at the same time.
I guess organic devs wanted separate appid for their 2 versions and fdroid just went with the default, original one.
So the 3 versions:
- Web: built by organic devs, you can download this from github releases, scraped from there by izzy
- Google: built by organic devs, you can download it from github or from play store
- Fdroid: built by fdroid build server, signed with fdroid devs' key.