The size the Designer displays your site on any particular breakpoint depends on your browser window size, however it’s worth noting that any change within a given breakpoint—whether it’s the high or low end—will affect all sizes within that breakpoint. When you’re in the second largest breakpoint you’re actually editing all styles between 1440px and 1919px.
You can change the size of the preview window in any breakpoint by hovering over the edge of the Designer workspace (between the two UI panels) and dragging left/right, or manually by clicking on the PX value in the top bar and changing the size as needed. If your browser can’t display the amount of pixels due to the available screen size, it will change the zoom level and show a number other than 100%.
I can’t say for sure why you’re seeing different numbers every time, it’s possible it could be referencing the last size it was on within a given breakpoint or you could be using the docked Navigator panel (which would reduce the horizontal width depending on it’s size) but I’d recommend paying more attention to the breakpoint level as opposed to the actual pixel value