Win 8.x security curiosities

I have a couple of networks set up, one a workgroup of several computers,
another for guest wi fi, etc. Up until Win8.x, computers could only "see"
the network routers unless they were part of the workgroup, in which case
they could see each other.

How is it that, though it isn't a part of that workgroup File Explorer on my
Win8.1 notebook is able to "see" computers on the workgroup? I've restricted
devices that can access the workgroup by MAC, so I'm not overly concerned
about security (and, of course, I can't log into those computers from the
notebook), but I'm wondering about the reasoning, as it implies that
networks with less security may be breached.

More curious, there are devices on the networks that the Win8 notebook
*doesn't* see, even though there are no specific restrictions to those
devices.

Insights are welcome...
--
best regards,

Neil