Spufford's book mentions this explicitly: during the description of one of the Central Planning Sessions one of the characters muses (i.e. provides an overview to the reader) about the many unwritten rules that guide each of the calculation steps. One (possibly the most important) is that "The Army always gets first pick to anything".

So the Objective Function assigned much higher "value" to Military production (either explicitly or implicitly) and this of course made military production more "efficient" than consumer goods production. (I put quotes around "efficient" because there still was a lot of waste and corruption even in that, of course).