Why did MPAC lower quarry and gravel pit assessments to values often below residential?

The recent decision by MPAC to lower assessments for gravel pits and quarries was the result of aggregate company appeals dating back to 2009, and a settlement negotiated by MPAC with the Ontario Stone Sand and Gravel Association (OSSGA), the representative of the aggregate producers in Ontario.  It appears that some Ontario municipalities, including Wellington County Township including Guelph Eramosa and Puslinch Townships, will appeal this decision since the impact on what is already nominal tax revenue is substantial.  If it stays in place, the new tax burden resulting from the change would inevitably be transferred to residents.