Why did two incompatible processes run together?

Hello,

We have the "Purge Concurrent Request and/or Manager Data" process scheduled within a request set.  We have many "Workflow Background Process" scheduled separately.  The two requests are setup as Incompatible at the Global and Program level.  They ran at the same time last weekend, and some of the "Workflow Background Process" did not reschedule themselves, but some did, and none reported errors.  Does anyone know how they were able to run at the same time?  I do not think they have to be incompatible at the Set level, the Program level should prevent them from running together.

Thank you, Kevin