Microsoft Office Activation - doesn't make sense?

I don't understand the Microsoft activation process.

I have Office 2019 - Bought with MS HUP - linked to my MS Live Account (I can see the license key in the portal for my account)
I have Office 2016 for MacOS - Bought with MS HUP - linked to my MS Live Account (I see a different license key)

I have 2019 installed on my Windows PC. Actually multiple PCs (even though the terms say I should only have it installed on one PC, it activated properly on Windows when I installed on second PC). Both have been activated for a year or more.

Then we bought a new Mac M1 and I downloaded MS Office 2019 from the App Store.
When I ran it it asked me to activate, and present me with a login. I didn't enter a key or anything. I logged into my MS Live account and it activated.

Now if I log into my portal, I see the 2019 PC key has activated the 2016 version of Office for Mac.

Does anybody understand why this worked at all? I'm not going to complain to MS of course but it seems that their activation system is really lax or something.