The cornea is not uniform. It is thinner in the center and thicker at the periphery, and even within small areas the thickness can be different. Different technology will often come up with different numbers. The cornea changes thickness with moisture and dehydration. Common medications can change corneal thickness.
The number to use for calculations should be the lowest number. That provides the greatest margin for error.
The thickness of the cornea is not the most important issue. What is important is the amount of untouched cornea after surgery. In a healthy cornea, at least 250 microns of cornea must remain untouched for the cornea to remain stable and more is always better.