Getting back to the original question:
All answers come with a few questions.
A1: This comes from a new feature in Avast. Let me say this; I used Avast for a long time and there are usually some fairly ugly bugs in early releases of new versions. It settles down after about two weeks and works well ... but yes Avast can be buggy. The fact that Avast gave you access to your router bothers me more than the the new scan result.
If I knew what parameter you changed, I or another member might be able to offer some advice. If you're too security concerned to provide details, then I'm afraid members won't be able to provide much help.
We know that you
have computers (how many?) that are running Win8 or 8.1
have one computer running Win10 bld 9860 (physical or virtual machine?)
the computers are networked (router, switch ??)
have an ISP (which one??)
have Avast 2015 (on all machines??)
Reacted to the results of a new Avast scan (which machine reported it Win10, one of the others?)
Clicked the "router" button
Changed some setting on the router (which setting?)
Ran the Avast scan again and got the all clear signal from Avast.
A2: No, although how the router and machines are configured with regards to the network does.
A3 Yes, it depends on configuring your network with high security and protections.
If you want help with network configuration, then post your network information on your thread. We're blind on this side.
On the Win10 machine, create a file showing how that machine is configured for IP.
Open a standard Command Prompt and type:
ipconfig /all > ipAll.txt
Attach the ipAll.txt file form your User folder to a new post.
Please identify the router make and model and
please state what configuration parameter your changed