Telephone calls to iPhone ringing on the Watch

I would like to better understand how the Watch and iPhone work together == I have an iPhone 12 Pro Max and an Apple Watch 7 and they are 'linked', such that if a call comes into my iPhone it rings on my Watch and I can answer it on both. I've always assumed that was because they were not too far apart.

But today what happened is I left my iPhone at home - 15 min/10-15mi away - and my watch was on my wrist. I was at a location where I was on the internet thru WiFi and working on my laptop [MacBook Pro running Mojave] - and a call came in on my wrist and I could talk to the person just fine == but my phone was far away [relatively] and I don't understand how/why the phone call could come into my watch without my iPhone at least being a bit close by. Thanks for any guidance.