Why does /All show different posts in different instances? - eviltoast

Like many others, I created accounts on a few different instances to try things out. Using apps like Liftoff and Jerboa make it very convenient to switch between accounts, but I’ve noticed that I get different posts showing up when browsing All, even with the same sort settings.

Can someone explain why that would be? I don’t think any of the instances I’m on are defederated, so shouldn’t I see the same posts across different instances?