About the same sense than a random pool swimmer, a random Japanse schoolgirl, a random gym goer / partime waitress, a random nun, a random blogger, a random housewife, a random gypsy, a random secretary, or a random biker chick, having out of the blue several deegres of resistance to the Dai Chu. Are they all coming straight from a game of Ars Magica? Naaah, they all have their resistances for plot reasons. And making another character resistant to any spell (be it the taxi driver to Transform, be it Leanne to Dai Chu after you free her but before you can actually cast the spell, - notice I saved THIS example for later) is always done for plot reasons.
TL;DR: Plot can justify lots of things
EDIT: Like, making the taxi driver resistant to transform PRECISELY for plot reasons.