@tim-s and @meyerdenney, yea I wish they didn't do that. Alexa's doing it now too, so I suspect there's some legal reason for it. I'm guessing it's to let users know this is a third party, not a Google service you're about to hear from.
I was research this and came across this pretty thorough blog post.
https://www.androidpolice.com/2018/02/10/ultimate-guide-google-home-tips-tricks-understanding-making-assistant-speaker/
I learned a few things from it, including that other people are annoyed by the multiple voices issue too:
"Assistant apps talk back to you in a different voice than the one you're used to hearing from your Google Home. It's jarring at first, but Home will respond with, "Alright, getting X," and another voice will come up and tell you what this particular app/service can do and will continue answering you until you're done with it and want to go back to the regular Home voice and commands..."
I also found this reddit post from a year ago that says Google Assistant will stop saying, "Alright, getting X" after you've been using X for a while. Unfortunately that hasn't been my experience.
https://www.reddit.com/r/googlehome/comments/80efhh/google_home_stopping_talkative_introduction_to/
I also noticed Alexa doesn't always say "Ok here's TRIGGERcmd, ..." For instance if I say, "Alexa, run calculator" she doesn't, so maybe if the command is short enough she doesn't say it.