Primal had a caching server in the middle that fetches from relays, then serves notes & notifications to you from there. You don't actually read anything from relays in Primal, though you do still write to them. It reduces bandwidth & creates consistency within their environment, but it can miss some events or take extra time to aggregate.
Alternatively, you might notice things missing in Nostria, on occassion, if it has difficulty connecting to either your relays or the relays of your contacts. This latter situation can affect any client. Many, like Nostria, use a list of fallback relays to try to reduce the effects of poor connectivity. Primal's situation is unique to Primal.
