Both of my Apple Watch's unlock my iMac, but they are both tied to the same Apple ID of course.
Are your Macs all set up with Family Sharing, same question for all of your family members and Apple Watch's? Do you have different user profiles on the Macs for each family member, each with their own password? Does everyone have their own Apple ID or are you all using one Apple ID (or some number in between)?
I'm thinking that all of these things may matter, and I'll be interested myself to see what others here have to say about this.