I think it is due to the size, or more importantly, the diameter of the cup. Where the pyrex is larger around, the anchor cups are smaller in diameter and slightly taller. Seems the heat would be concentrated more. Pyrex cups are somewhat thicker though and definitely hold the heat longer as Willy stated.
After viewing some of Bass8baits incredible 3 and 4 color pours, he told me those were the only cups he used. I bought some right away and prefer the anchor over the pyrex unless I am using a little over 8 oz of plastic.
Wish I could pour like Javier though as it was NOT the cups!!!
Jim