Nitro XV282K KV screen size 2560x1440? What gives, exactly?

Okay, I may just be a complete dolt, and if so I don't mind if someone tells me. I just bought the Nitro XV282K monitor, it's 28 inches, 144 hz refresh rate and 4k resolution as my main monitor. Before this, I had an old 1080p 60hz monitor for the past forever. The only time I messed with anything higher than 1920x1080 was DSR factors in nvidia control panel, and I rarely ever did that so I never really had experience with higher resolutions. I say all this just so you know why I might be stupid when you read my question. It's an excuse, if you will.

Anywho, games keep defaulting to 2560x1440. When a game is in windowed fullscreen mode, it is defaulting to 2560x1440 and won't let me change it. I can visually see a difference in fullscreen when I change from 2560x1440 to 3840x2160, but every game I play is defaulting to 1440. I also use a program called Windowed Borderless Gaming, which if you use you already know basically just removes the border when in windowed mode and lets you change the settings on an application by application basis, in case the default isn't correct. I tried this for an older game (Dragon's Dogma: Dark Arisen) and when I added it to the program to be made borderless, it was zooming in to the top-left of the image, like when I would set a game to 3840x2160 on my old 1080 monitor and it would stretch out because that doesn't fit. So I had to change the settings in Windowed Borderless Gaming to make the window 2560x1440, and it fit perfectly.

To test this, I went to https://www.whatismyscreenresolution.org/ which shows info on your screen size and resolution, etc. And it says at the top "Your screen resolution is 2560x1440." Where it says "Device resolution," it shows the correct 3840x2160, but if I move the window to my second monitor (which is still a 1920x1080 monitor), it correctly says at the top "Your screen resolution is 1920x1080" as well as saying 1920x1080 for "Device resolution."

What gives, exactly? I have quintuple-checked both the nvidia control panel as well as Windows settings and they both have my display set to 3840x2160. It shows that is the native resolution. So why do games keep running in 1440, and that website show 1440 as my resolution? I began to wonder if this monitor is actually just 2560x1440 and upscaling to 4k, but that couldn't be the case, and I must just be crazy, right?

 [Edited the thread to add issue detail]