They are a measure of your progression in each energy system, not an absolute measure of fitness. So as you complete more workouts for each energy system your progression levels will rise. Someone with a 3 W/kg FTP could be at a progression level of 8 for Threshold, while someone else with a 5 W/kg FTP could be at the progression level of 3 (say).
Here’s a nice summary: Are Progression Levels just new name for Training Zones? - #4 by SeanHurley