I have noticed the same problem too, I develop some websites, and one that I am working on recently has been specified to be 930 pixels wide, this then overflows a little bit.

So, I suspect that what happens is that the browser defaults to 800 pixels wide (the width of the screen). The browser tries to diaply the website at 800 pixels wide (just lke any other borwser if you set it to that width), but as the minimum size of the website is set to more, it overflows.

It would be nice though if the browser was a little bit more intelligent, and zoomed out a bit if the website was too wide