That must have something to do with the way you've got contacts saved or the app you/re using. If someone texts me, I swipe over and hit reply, speak my message, and it sends it to the number they texted from automatically unless I tap cancel before the timer runs out.
That's using hangouts. FB messenger has similar functionality. However, to initiate a text from the main screen of the watch, I simply tap it or say Ok Google, Text Holly, I'll be a bit late tonight (or whatever the message is). However, I've just about only ever done THAT with contacts that only have a cell number. Can't recall whether or not I've used it to text someone like my parents, who I have multiple numbers for because they insist on keeping their landline in case of emergency. Again, having a specific number set as "primary" or default might make all the difference there.