very few people want a browser running 1920px wide, it just isn't a good use of screen space and nor does it give good results on a lot of websites, so they are more likely to have two windows side-by-side..
Because having two windows side by side is so useful on screens that wide Microsoft introduced keyboard commands into Windows 7 that allow you to easily swap the windows around. Windowkey+leftarrow makes the corrent program window resize to exactly fill the left half of the screen, Windowkey+rightarroew makes it exactly fill the right hapf of the screen and Windowkey+uparror restores it to its original size.
Using this on a 1920 wide screen gives you two windows exactly 960 wide each and gives you a browser viewport width of 958 maximum (assuming that your borwser only takes one pixel each for the browser borders themselves and that the page does not have a scrollbar. In practical terms that means a web page will usually be provided with somewhere between 940 and 950 pixels in which to display - which gives reasonable line lengths for reading the content even if the page has only one column.
Of course most people using some device other than a computer will not have a screen anywhere near this big ans so probably will have the browser maximised within the perhaps 300 or 400 pixel width that their device provides.