No two houses, even built by the same builder to the same plan, are truely identical. A manufactured house made on an assembly line might be really close. So, what happens in one house is no guarantee the same thing will happen in another, especially since people don't react the same. You might have a larger hot water heater and regularly take longer showers/baths/etc that run it until cold. This simple fact, vs a neighbor being more conservative and only taking s short shower could be enough so that the expansion and resulting pressure increase doesn't trip their T&P valve. Your check valve may actually work better than theirs resluting in a perfect seal, theirs may leak - it doesn't take much - take a look at how much water comes out when it does leak - it is not a torrent, or if it is, it doesn't last long (that is my guess); maybe a few ounces?
Sometimes, it just is.
As I said, with a closed system, they invented the expansion tank, and one should have been installed by common design practices on your house because it is a closed system. The only way you could get long term reliable results without one would be if you had really low incoming pressure so the resulting rise didn't reach the appropriate level, or if you had a tankless system so there was no volume of hot water to expand unless you were actually using it with a valve open (at least those are the only ones I can think of off the top of my head).
Also, I'm not a pro, but there has been more than one person here recently that fought the situation like you, installed one, and it fixed the problem. A tripping T&P valve is common in a closed system without a pressure tank. Bad T&P valves happen, too. Have you had several in a row, possible, but not really likely. The cause and effect in your situation is closed system, no expansion tank. A (unlikley) bad T&P valve is probably not. I really like to know why about things, too. In this case, I'm satisfied - but I'm not you. You can keep looking, but my suggestion is live with it, add a tank since there should be one anyway, and if that doesn't fix it, then you've got a problem.