Mirror modifier changes behaviour of scaling 2.8 I think I'm just doing something wrong here. I looked around here and found several issues pertaining to scaling and mirroring but I couldn't find one that I felt was applicable to me.

Basically I'm learning Blender for the first time through an online course. They had us cut one of our models in two, and then apply the mirror modifier.

This works fine, but then I try to scale out the top ring of my model by three units, and got some weird results.



The top one has a mirror modifier with the top ring scaled out three units and you can see how it's created this oval shape.

The bottom one is the same shape as above pre-scaling. I applied the mirror modifier so it became a full shape again, and then selected the top ring and scaled it out again by three units, and it presents how I expected the mirror modifier above to.. So what am I missing?

You can find my .blend file here: https://drive.google.com/file/d/1rSZ_AsEMzvMw5ZtUSTj-y5NMIoj8Fer0/view?usp=sharing
If you use median point or individual origins as pivot point as you can see on the first picture the transformation's origin won't be at the center of your selected mesh because it doesn't include the mirrored verts only the real ones. So you have to use the 3D cursor as the transformation's origin at the right place as you can see on the last picture.