That means they're adjusted their antennas to give coverage to another "spread" of area, and your house is, simply, unlucky.
Edit: to prove my point, this is at my previous house - it goes to show that even though the signal strength isn't strong enough, there isn't enough external interference (other radio waves, jammers, antenna placement) to ruin the performance.
I don't know what it is these guys do differently than the other operators but it could be the antenna tilts/coverage preference/interference from adjacent cells... Not sure. And maybe there's interference from other networks, or just lots of concrete, or trees. Or maybe the other networks have reduced their transmitting power (one requisite for spectrum reuse) and perhaps that's happened in your case, in your antenna. Really can't say.
If it doesn't work for you, you're free to switch over to another network.
Edit 2: same house, upstairs. Look at the better signal strength and worse off performance.