HTC seems to have different models with different screen brightness and as a result also different battery life!
I did some research and it now seems pretty clear that HTC is using two different screen types in the One, just like it did use both Acer and Sharp in the One X. The Sharp had the higher brightness and higher color temperature.
The same pattern can be observed for the One so far:
- 26.02.2013 RUS 3dnews.ru: 382cd/m² and 1,258:1 contrast | 6,938K color temperature
- 14.03.2013 GBR uk.hardware.info: 426cd/m² brightness | 977:1 contrast | 7,820K color temperature
- 14.03.2013 NLD Tweakers.net: 489cd/m² | 1159:1 contrast | 8,166 color temperature
- 19.03.2013 DEU Notebookjournal: 385cd/m² and 1100:1 contrast
- 20.03.2013 DEU Computerbase.de: 483cd/m² brightness | 1100:1 contrast | 6,600K color temperature
- 20.03.2013 GBR pcpro: 481cd/m² brightness | 1202:1 contrast
- 20.03.2013 DEU Chip.de: 479cd/m² brightness | 1020:1 contrast
- 23.03.2013 USA GSMArena: 647cd/m² brightness | 1541:1 contrast
As can be seen. there doesn't seem to be any pattern in terms of date or market so far, with both screen versions appearing in German reviews. Perhaps because of the Apple competition in the U.S., the screen in the US review gets higher brightness.
So this also explains why battery tests have been a mixed bag. It will depend a lot on the maximum brightness of the screen on your specific HTC One version!