I got a 98-percentile result and realised my Mac had Night Shift turned on.

And what if you are wearing blue blocker readers? I am, and perhaps unsurprisingly my result was greener than average.

ETA: But of course when I retook the test without my glasses, I went even greener.

Yes, this test only works somewhat if you have multiple people use the same monitor/screen. Otherwise it's mostly useless for accuracy.

All iPhones are calibrated.

Maybe both are true, if someone grows up and learns through a specific monitor, maybe that will influence and define their blue definition.

followed by the number of rods and cones in your eyes, as well as their own sensitivity, as well as your language

Exactly, and how bright is your display compared to your surroundings at time of viewing?