The signal quality you are used to observe is meant for Infrastructure Mode, when you are connected only to one point: the Access Point. And that indicator shows the signal quality between you and the AP.
In a mesh network, however, your station may be connected to several other stations at the same time, thus it's hard to show one indicator representing all of them.
Different manufacturers decided differently on how to report the signal strength in such a case; the smarter ones report the average value, and the cheapest ones always report a fixed signal strength no matter how weak or strong the actual signals are.
Try to update your adapter driver, there are chances that a newer driver represents signal strength data somehow closer to the reality.