It's not listening to actual words, that's already too complex (you'd have to parse language for that, which those low power chips can't do). It's listening for syllables, Oh-Kay-Goo-Gle or whatever. Depends on the chip and implementation of course, which is also why you get false positives when someone says something similar.

If you add more syllables to that then your phone would activate literally all the time, with tons of false positives.

Seriously, if we had low powered voice recording + voice to text you'd already have instant conversation subtitles on your phone, instant translation and so on. We simply don't have that yet, those features do exist but they are power hungry (so if you do use them say goodbye to your battery life).