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Difficult to explain the provided "correct" answer, as it's, IMO, rather a poor answer for "why".
I assume it's trying to emphasis that, as it describes, the WLAPs don't actually provide host addresses. But as the wireless VoIP phone shows itself on a different subnet, this doesn't really explain "why" the two PCs are on the same subnet, as they don't need to be.
Perhaps a better explanation might be that the two PCs are on the same subnet as the shown server. From this, one might assume there's much traffic between the servers and PCs in the same subnet, and also perhaps no additional security (like ACLs) restricts the traffic between those PCs and the server. I.e. it mimics a common wired network.
The wireless VoIP phone being on a different subnet might be significant, as it's likely on a different SSID/WLAN so it can be provided different wireless QoS.
Again, the "correct" answers looks to be about the best of the possible answers provided, but might not be really the best possible answer for "why", as the PC hosts could each be on different subnets too (which could also be different from the server's subnet).
