Manage has limited ability to control fixed overhead costs in the short run because these costs are incurred to provide capacity to produce. fixed cost can be controllable to a limited extent at the point of commitment,therefore the FOH spending variance can be considered in part controllable.
on the others hand production levels are controllable to a very limited extent in the production area. production is more often related to ability to sell and demand thus these levels are not controllable by the production manager. That,s why fixed overhead variances considered non controllable.