It's a typo. The ork book used to list both bikers and nobs in the warbikers entry as having 1 wound. This was errata'd to the nobs having two wounds. When they re-printed the book, they put the bikers and the nobs as two wounds, but they havn't chantged the errata to show that regular warbikers are one wound.
I have the old copy of Codex: Orks, but unless they really REALLY screwed up, page 46, the warbikers bestiary entry, should have the correct wounds listed there for warbikers. Point the dude to that page next time you see him. Anyone wanna confirm that page 46 has the correct number of wounds on it for us? Thanks to whoever does in advance.
Edit: Spelling!