https://bugzilla.kernel.org/show_bug.cgi?id=54381
--- Comment #7 from Alex Deucher alexdeucher@gmail.com 2013-02-25 20:58:37 --- 1920x1200 (0x70) 154.0MHz 1920x1080 (0x72) 148.5MHz
These two pixel clocks are close enough that your monitors seem to be able to deal with the slight difference. Unfortunately, not all monitors are so forgiving and and it wouldn't work if you picked a mode with a substantially different pixel clock. You just happened to get lucky.