The description at the top says "Voice: 90% cell edge probability, 50% cell loading factor, maximum resolution of 100 meters. Data: 5/1 Mbps, 90% cell edge probability, 50% cell loading factor, maximum resolution of 100 meters."
Basically, it's a probability gradient under certain conditions. You can be the 10%.
This wouldn't surprise me at all. I ran into the same issue with AT&T during the switch from 3G to 4G. Suddenly, signal at my house went to absolute crap. Couldn't make or get calls. Texts would never send, texts would arrive in a random burst overnight.
Repeated visits to the AT&T store to complain were met with them pulling up a google maps like coverage map and saying "Nope see you have fine coverage at your house, go away please."
I'd guess the map is computed based purely on cell tower location+geography.
Which, I'm reading your comment from a device which is capable of connecting to all four carriers represented, and it's connected to the Internet through wifi and one of those four carriers. Why doesn't this map have any user submitted data, eg from the FCC speed test app. https://apps.apple.com/us/app/fcc-speed-test/id794322383
I wonder how accurate they really are.