I am not an expert in the process of this test, but here is what I do know:
The measurement of heat really cannot be performed using an exact science. Since the capsicum molecule affects the taste receptors of each individual differently. A "Scoville" test typically involves a controlled set of individuals who are subjected to tasting an amount of capsicum product and measured by how much solution dissapates the heat. Again, since each individual has a unique configuration of taste receptors, the results are averaged. Someone else could invent another test using some other means of measurement but Scoville's method has endured for a century as the standard measurement. Labs can extract capsicum from products and measure content proportionally but this does not provide the perceived heat level to a taster. The sugar molecule is a good substitute to replace the capsicum molecule after it is lifted from the taste receptor. Through a process of rinsing with the solution, the capscicum is eventually replaced with sugar and the heat perception is eliminated.
I am actually curious as to how a lab test is conducted with measuring the Scoville level of a Red Savina Habanero :) I would not want to be in that taste testing group.
Hope this helps,
Firetaste