This odd sounding announcement seems to be only for the split tracking during a walk/run (that I've noticed). It also seems to depend on which Siri voice is used on the watch. I've only noticed it on American - Voice 4.
Thanks for reminding me. Yes, there are choices for Siri's voice on the Watch. I prefer the one that is closest to the original, which appears to be American - Voice 4. That's the one I'm hearing the oddity in. I changed up to American - Voice 2 for my morning walk and sure enough, that voice is normal. But since I don't care for that voice I'm switching back.
It's so odd. I remember before the change, Siri would misspeak that announcement in a different way (but I don't remember how). Oh well, it is the way it is, at least for now.