Their brain crystal could be measured, but there wasn't a metric to rank them. The biological supercomputer gave a rank to powers because it was using the customs humans used, based on the information it got from Erik when it parasitized him.
But the measure of a power's might, of their efficiency, rested only on the number of neural links, meaning that ranks were just something the humans used to classify their enemies.
This classification system, although arbitrary, helped humans understand and prepare for threats. By having a standardized way to measure power levels, they could better strategize, allocate resources, and assess risks in combat situations. Even if the system wasn't perfect, it provided a framework for tactical decisions and threat assessment.