Im not enterly sure. Im not an expert in the protocol but my guess is mabey a diffrence in contact and relay lists when you hop between apps.

Sometimes ill install a new client to play with and it will have a hard time pulling what I consider my "main" set of relays and will have their own sets of relays that might not have the full set of people I follow or an outdated set. so when I swap back to an app I use a lot ( amethyst on my phone) ill notice that im following people that I unfollowed and am missing recent follows.

Thats just a guess though.