I noticed this also, that updates arrive with different timing for different devices, even though the different devices are supposedly running precisely the same OS, PureOS Byzantium.
Just speculating, but it could be that some updates are intentionally spear out over time, to avoid a sudden disaster on many devices at once. If a certain update leads to a terrible problem then spreading users out over time means that there may be time to roll it back before all users are affected. Just guessing, no idea if this is really what is going on.