Based on this blog entry
and it turns out I had mis-measured, using my iPhone as a rudimentary tester. This is a good demonstration of why that isn’t a great idea, the consistency just isn’t there. The Apple Airport utility, which is the only way of getting an RSSI reading out of iOS, can be very slow to update and becomes especially problematic if your device roams to a different BSSID.
As explained in the video, this test using the Ekahau Sidekick is better but not thorough. It’s closer to the real world scenarios I have to deal and confirms my house does have surprisingly solid walls.