For a couple of contacts, Siri chooses to send an SMS to his email address instead of calling his phone number. It seems to be contacts that I have both an iCloud and Exchange contact for, and where these two contacts are merged.
These contacts are people with whom I already have threads in messages, and in 90% cases even iMessage users. Nevertheless, for some reason, if I ask Siri on Apple Watch to send a message to these people, send it and SMS to their work email address.
I'm not sure, but it can be a link to when using Apple Watch on mobile instead of in Bluetooth range for my iPhone – but I'm not positive about this.