Because they aren't standards. 1080p, 720p, etc, actually mean things (1920x1080, 1280x720, and so on). "900p" and "792p" can only be assumed to mean 900 pixels and 792 pixels on the Y-axis, and nothing about the X-axis. It could be something like 1200x900 and 1056x792. Of course it's "p" because you can't make interlaced games.