You mean go backwards to 1.266, or go 1.266 over 1.8? If you're referring to the latter then yes it could probably be done and maybe higher, but it could cause system problems and you would probably need a water cooling system to disperse the extra heat, and the CPU will probably also have a shorter life, not by much though if you cool it properly.
I don't know what it means, I figure it's just PC Probe being PC Probe (aka stupid ) It may be the max speed supported by the chipset you have but as it's under processor who knows. I really doubt it's how high you could overclock unless the Asus engineers who wrote the software are very optimistic.