I'm non-CF bronchiectasis and take a PFT or spirometry whenever I get antibiotics. (Sorry to post here since I don't have CF).

From what I understand of it a healthy person's lungs should be near 95-100%. It's not like running a 5k, because there you're comparing yourself against others. Your "score" there can be determined by being first or last, mean, percentiles, or an outlier, etc.

With FEV1 you're measuring how well you clear the air out of your own lungs. So the reasoning works backwards- if you don't have a good FEV1, then you don't have healthy lungs. If there's mucous, like CF and non-CF bronchiectasis, the techs explained it like whenever you exhale hard your body involuntarily treats it like a cough if it detects mucous. It's as involuntary as breathing and coughing itself and you really don't have control over it. So the air flow graph has a fluctuation in it if there's any mucus and it wrecks the FEV1 score.

I remember one time I took it to try and pass a physical and it was a very slow day so the tech let me cheat. I was there for over an hour clearing my lungs, postural drainage upside down, taking meds and drinking lots of water. I felt like my lungs were VERY clear after a times going back and forth. But no matter how much I hacked and spit out the mucus, when I kept returning to the spirometry there was always still mucus and there was always a very subtle cough/hesitation only the machine could detect.