We prefer to guarantee that you will get 100% full coverage. That guarantee doesn't hold either side liable for quantity of heads.
Assume we say there will be 50 heads but we only put in 49, we do not want to have to give rebates and we do not want arguments at the end of the job. On the other hand, assume we say 50 heads are going in but we use 60 because when we showed up you installed a patio that now required us to go around the patio using different types of sprinklers. We do not want to have to ask for additional money for the job and we know our clients wouldn't like if we did that. So again, eliminating quantity of heads guarantees that both sides will be completely satisfied when the job is done and their will no surprises.