OK, guys,
To sum up the facts:
BEFORE GDR3, WP8 IE used to perform (or act) as being full-fledged 720x1280/1280x720 device (where pixel means actual "atomic" pixel, also known as
hardware pixel).
AFTER GDR3, IE apparently switched to so-called "
reference pixels", which, from a developer's point of view, are a mixed bag of blessing and a curse, causing the "Where TF my pixels have gone" effect... And 338 x 601 px or so resolution reports from majority of online check-my-screen-res tools...
For those who care, there are dozens of articles explaining the matter (
A pixel is not a pixel,
A pixel identity crisis,
Optimising for High Pixel Density Displays, to name a few), but for the developers caught by (GDR3) surprise it actually means more labor to achieve the same effect as when good-ol' hardware pixels were THE thing.
As I have said earlier, for average day to day user, it doesn't mean a diddly-squat, but for responsive-design developers it introduces a new universe of variations into the equation... Not to mention the Android's DIPs (device independent pixels) and other device-specific nightmares...
@luk3ja: Recognizing the device(s) type has absolutely nothing to do with screen resolution! It's done by a plethora of different methods (UserAgent strings and/or browser specific properties exposed, among others) and that's why you have an option to introduce your browser as "desktop or mobile version". So it's really not necessary to involve pixel-or-not-pixel shenanigans. With the latest development in hardware industry, the borders defining desktop, laptop-notebook-netbook, tablet and smartphone devices are becoming more and more blurry, tending to disappear in the nearest future (remind me of this in, let's say, 2016[SUP]th[/SUP])... We, the proud Win8/WP8 owners/users should realize that, after all (OK, I am a tiny bit sarcastic, over here)... So, should we cheer MS tendency to adopt some half-way-half-assed-compromise-transition-period standards or should we support the developers in providing the quality app designs without being bothered with all that. The only consequence of "fixing the bug", as you've put it, is engaging developers all around the World in additional coding to address a sudden "resolution change" in already good working designs. Luckily, we all manage quite nicely with (r)em-based fluid designs (at least the "enlightened" ones) but, every now and then, it's necessary to take a pixel-first-based-percentage path...
The point is - of all the websites/portals/blogs/RSS feeds/podcasts/and-such introducing the latest WP8 update, there was precisely NONE to warn us about this, and that's what got me into this debate.
By the way, YouTube went bananas, too - the playlists and all, with some video containers performing John-Hurt-moments on their DOM parents. Is that an upgrade, too, hidden as may be? I predict GDR4 or 8.1 or Blue or SP2 or whatever to fix (back) IE to some senses... :winktongue:
I've tested on Android, iPhone and iPad and ALL report lower resolution than their actual screens resolution because this is how mobile web browsers work so that they can display things in an easy to view scale. For example if you had a 1080p screen on a 4 inch device everything would be TINY and websites would think you were on a laptop and would refuse to display the mobile version of the site so it reports itself as a lower resolution to stop this from happening and to let mobile websites know that it is a mobile device. I'm glad they fixed it! 
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