If each frame in the media contains the exact 1920 × 1080 pixels ...
This image has the same number of pixels on the top and bottom half, but you can probably see the bottom half looks worse. That's what lower bitrate does. It's like turning up the compression on a jpg -- you are not getting the exact same pixels, just the exact same image size.
Once you have an account, either because you got an invitation or reg is open, most usenet indexers have a free tier that lets you grab a few files per day. The invitation alone is not going to give you a premium account for free.
I do recommend getting on to a couple of good indexers and using the free grabs to see how you like usenet. Personally, I love it, even though it isn't free. YMMV.