In analytics data, IE8/9 appear to be reporting different, sometimes non-standard resolutions from screen.width / height on the same device, for instance: 1140×641, 1366×768, 1249×702. Those variables are supposed to report screen resolution, not something flexible like browser viewport size, and so I would expect them to always be fixed.
Is it possible that the data I have is accurate, i.e. that these could all be from a single device? Or am I without doubt actually observing three different devices?
Googling reveals this and this which are examples of IE8’s screen resolution reporting being inaccurate, but do not explain why the values might change over time. Are there other known issues with these variables?
Background is here, the screen.width variable is being called by Google Analytics code on user machines. I’m trying to understand the anomaly by eliminating possibilities as I think of them.
In fact MSIE and Firefox adjust the physical screen resolution with the zooming factor being applied in the browser into the screen.width value.
This is why you get odd dimensions for MSIE and Firefox, but not Safari nor Chrome
See also How to detect page zoom level in all modern browsers? about detecting zoom level per browser.