It's not just the cooling block; little things like rough surfaces, voids in the thermal paste, or too much thermal paste can make a huge difference in the cooling performance.

Tech Ingredients has some detailed discussion in the linked video ~28:20 regarding surface preparation and paste application. (Lasers not CPUs, but the principles are the same.)

https://youtu.be/9VmITd0dKAo?t=1700&feature=shared