Access Point channel allocation and channel mapping often is more important than raw WiFi signal strength.

Assigning WiFi channels to interleave with each other reduces interference and setting the channel overlaps to physically distant routers can make a big difference.

Do you use an app like this WiFi Analyzer to see what WiFi sources are using which channels and manage the frequency overlaps?

I often find a huge number of w24Ghz WiFi sources 'stacked' onto the same channel, sometimes a half dozen or more on channel 6, for example.

Even on the 5GHz band it is not unusual to find several routers using the exact same couple of channels rather than spacing themselves out. 5GHz band has some weirdness with some channels being regulated to lower power and some channels only being available in automatic mode, not manual settable.