Assuming PZ is the class for (Polaris) Business Class upgrade using pp or Miles+Cash, why sometimes it shows PZ# where # is not 0, but Flight Status still shows that peoples are in waiting list?
Either it's a CPU flight, and you're seeing pax waitlisted without instruments, or the upgrades just haven't processed yet. UA does not process upgrades dynamically which means there can be a lag of several minutes to several hours after PZ opens and before upgrades process. This is why it's possible to snipe an upgrade ahead of pax who are waitlisted.