According to the description quote "....reduces their depletion by 2% per level...."
I just upgrade Level 12 do Level 13. My Metal Mine Depletion before the upgrade, Level 12, the Depletion is at 1.51%. After the upgrade to Level 13, the Depletion is at 1.47%.
Is this a bug? Should the Depletion 1.51% at Level 12 reduce to 0% or normal once it goes up to Level 13 as per the description for the Pumping Technology?