Character's general proficiency with Japanese also affects it. Some characters won't be affected by that like Rui or Sato, but ones that have lived overseas primarily or aren't from Japan originally will normally score a little lower than they normally would. Or a lot lower.

Although now that I look back I don't think Alexei was meant to be that low. I rechecked his background and yeah it should be higher. That should be fixed now.

Kotori got perfect mark because she BREATHES STUDYING. Seriously.

I'll be sure to do more thorough background checks for next exams.