When I took that test in real life while I studied to become a certified print technician 3 years ago, I got a perfect score. On this uncalibrated POS screen that I have no idea or desire to calibrate to printing specs, I got a 24...
Boredom also played a factor since I just sat there, thinking "I gotta go soon, this looks close enough!"...
I really think it´s down to your monitor, and that can play a factor either in the single digits or double digits for older monitors. I know monitors in the printing buisness have to be calibrated like hell so that they portray color as close to real life as possible, even simulating different types of paper. I´ve done it once, it´s tedious work.
While not defending that score, I don´t think it means anything when you do it online. Do it in real life, with real wavelenghts entering your eyes, instead of a mixture of Orange-Red, Green and Cyan-Blue diodes, faking the illusion of color.