Without knowing the specific data item collected, browser sizes can vary given constant screen resolution. Just a few guesses off the top of my head: Font size

Without knowing the specific data item collected, browser sizes can vary given constant screen resolution. Just a few guesses off the top of my head: Font size for the window title-bar, for the bookmark bar, for the tabs. Browser toolbars, whether or not title bar exists, whether or not bookmark bar is enabled. Whether or not window title-bar is enabled. Status bar, task bar (could be at the side). Two windows stacked vertically on a portrait monitor. And so forth.