The speaker doesn't understand "what you're saying" - it listens for key vocal frequencies that more or less come together to form the expected phrase "Ok, Google" - Assuming you have not fully voice trained your Assistant, it has to fuzz its listening expectations, as it doesn't know your specific voice frequencies that correlate with "Ok, Google".

What you end up with is a system that's listening very broadly for something that sounds like "Ok Google" and with the amount of fuzzing needed to capture that, "good boy" can come close enough on key frequencies to match up with "okay" and then any further speech might match up with a fuzzed expectation of "google".

Think about it, some people repeat phrases to their dogs a few times, "Who's a g__OO__d bo__Y__? wh__OO__'s A good boy?" (cap'd and bolded for fuzzy syllables probably recognized)