It's fan comments gone wild.
In one slide, HTC showed a picture of a Bayer filter. That was by the marketing guys, and it was a concept graphic to sell the camera. I expect that that was a first for many people.
It's a 4.3 MP camera, according to good sources (4, 4.3, whatever, same thing for marketing).
Eager fans looked at the Bayer filter picture and decided somehow that it had 4.3 MP each of red, blue and green and 4.3MP times 3 colors = 12.9, aka 13 MP.
The math and the rest of the Ultrapixel briefing confirms that it's a 4 MP camera.
With every new phone feature, the blogosphere scrambles until fan assertions become facts, whether true or false.
That's my opinion.