Last month we saw the announcement of the new Lumia 920 and 820. Back then, there was some concern for the 920's battery life. Having such a big screen, HD resolution, a dual core processor, not to mention LTE, it really needed a powerful battery to keep it going through the day. The 2000 mAh that it came with seemed to be the perfect match.
Nokia touted 10 hours talk time over 3G with 400 hours in standby mode. Playing music would also get you about 67 hours of uninterrupted listening. However the AT&T version has downgraded battery time estimates. According to the product page, the 920 only gets you 320 hours of standby, 9 hours of talking and only 52 hours of music. The same goes true for the 820, that also lost a few hours here and there.
The reason for the change in specs is unknown. It could be that after carrier testing AT&T asked Nokia to tune their firmware in some way resulting in less battery juice. Or it could just be that Nokia and AT&T are using different tests and different standards. What's even more interesting is that the international versions of the phones still retain their initial specs as you can see on Nokia's product page.
Of course there's even a chance that these AT&T specs will be revised again when we get closer to the launch.