Bulk Plan Milestones different in Test Environment for Same Order Set

I have noticed that our Production Bulk Plans spend the majority of the duration in the BUILD SHIPMENT GRAPHS COLLECTIONS Milestone.  Wanted to do some thread increase testing in our Test environment (which is a very recent copy of Prod) to understand the efficiency impacts that might have.  However, in testing, I notice that the Bulk Plans in the Test environment spend more time in the BUILD SHIPMENT Milestone rather than the BUILD SHIPMENT GRAPH COLLECTIONS Milestone.  So my question is: can you think of any settings that can influence why a Bulk Plan would process the same order set