If they were 24" OC, would you not be 12" from each truss, measured to the centerline of the head, if the head is centered between the trusses?
You have to think of this in 3d. By looking at a plan view of a 16" OC truss layout, then no, you couldn't get 10.5" away. However, the fitter will hang the line through the opening in the truss, so your measurement would be taken diagonally from the nearest member. If the truss is 18" deep as you state, call out your branchline el. at 9" below the deck. Running through the widest open space, you'll be 8" horizontally from each truss, plus the diagonal distance from any member.
I can agree that words alone are hard to interpret. If you want to sprinkle between every truss (sprinklers spaced at 16" intervals!!) be my guest. Even if you used the QR reduction, and you got the design area down to 900 sq. ft., you would have something like 50 sprinklers in your design area, all flowing a minimum of 7 psi. Ouch.