The scenario is the protagonist in this novel had been experimenting a theory her colleague have had told her (is the tense correct?) that eye contact with another person for more than six seconds without looking away or blinking revealed a desire for either sex or murder.
She reflexively hadn't believed this, but it had intrigued here enough to test it out on various friends and strangers. To her surprise, with the exception of John [her husband], one of them always looked away before the six seconds was up.
– Cited from Still Alice
My question is, why did the author use "one of them" but not "each one of them" or "every one of them"?