While it is true that the mash temp does have an effect on the amount of fermentable sugars and the resultant FG (and apparent attenuation %), the OP is performing a split batch (same wort, regardless of mash temp) with two different yeast strains fermented in the same environment (temp controlled, slow ramp upwards, etc.). The independent variables are the yeast strains (500 vs 530), the significant variance in the dependent variable (the SG, or FG) between the two strains while using the same wort and fermentation conditions is the mystery. One would assume that since the WL500 was able to ferment down to 1.012, then, since it was pitched into the exact same wort and fermented under the exact same conditions, why didn't the WL530 strain, as well. The mash temp shouldn't affect this variance we're observing. Any effect the mash temp of the wort has on the FG of the beer should be realized with both yeast strains, i.e., T=147F might yield FG around 1.009 while T=156F might yield FG around 1.020 for both yeast strains unless there's a significant difference in apparent attenuation bteween the two strains.