Halo 3 Achievement question?

[spartan117]

hey guys,
i have all achievements in halo 3, but when i was playing coop on legendary, when i finnished, i got this achievement called "the road"
but i had all achiements, 1000/1000 gamer points...
and when i checked the list on my xbox 360 dashboard i didn't see any achievement called "the road"...
what the hell?
what kind of an achievement was that???

 

[Hellboy]

You get the acheivement "the road" for completing the 3rd level of the campaign on any difficulty. You get 20g's and the little icon for it is the roman numeral for 3 (III) on a blue forerunner circular icon.