Another confusing paragraph was the initial count off of the Ork defenders at the refinery. (emphasis mine)

"There are eight Ork Slugga Boys (see page 239) led by a Nob (see page 239) and five Gretchin led by a Runtherd (see page 240) defending the facility. One Ork is assigned to each of the nine watchtowers at all times. The remaining twenty-one Slugga Boys…The Nob rests in the Living Quarters at night and bosses around on the remaining Orks during the day time."

How do we get  (8 boys + 1 nob + 5 Grots + Runtherd) - 9 Boys (in towers) = 21 Boys

By the initial count there is actually -1 boys in the towers….

Should it perhaps be….

"There are thirty Ork Slugga Boys (see page 239) led by…"

This is all on page 257 btw.

Danayel said:

Math seems fine to me. The currently active squad is 8 Slugga Boyz, a Runtherd, and a Nob = 10 Orks. 

21 additional Boyz are not actively defending the walls, and goofing off within the camp, but presumably get occasionally subbed out for the active defenders. 

There are 9 towers, one for each of the active Orks, with the Nob supervising. Notice it says "One Ork is assigned to each watchtower," not "One Slugga Boy."

However, I guess it would be more clearly worded if it said 30 guarding the facility, but only 9 actively guarding the walls at any given time.