A variety of possibilities. First, I find it a little suspicious that you get the same(ish) signal reading in such different parts of the house. I suppose one explanation is that the router is dead-center of the house (good). However, another reason could be extenders/repeaters. One thing about extenders/repeaters though is that they’ll always give high signal strength but will frequently reduce bandwidth – sometimes by a lot.
Also, if you’re using a combined network (ie: one SSID for 2.4GHz and 5GHz) then "excellent" may not be the same excellent as they could be on different bands.
And you could have neighbors on the bad side of the house who are using the same Wi-Fi channel. Your signal may be at an excellent level, but so might theirs. Around here I've noticed that some ISP installers are pretty lazy/cheap and install routers where the cable comes in - meaning their neighbors get about half of the Wi-Fi signal.
Or, at the bad end, there could be some RF noise of another sort.
Of course, I guess this could also mean that the terms like “excellent” are not very precise.
