The picture on the left is not correct though.
It is, actually. Not to start a pissing contest but my range of SCART TVs, RGB hacked TVs, arcade monitors, PCBs and GroovyMAME setups will attest to it. Besides which, you can take SailorSat's word on anything related to CRTs and image quality.

GroovyMAME is showing the image true to the original. The problem for you is that you seem to prefer a distorted, softer image than the original RGB progressive scan image. It's a personal taste thing. Some people like the razor sharp look of a Sony PVM. For me, it's too harsh. I prefer a consumer grade TV or arcade monitor because they're softer and have more image artifacts. It seems that you prefer something even softer than that.
If you never noticed the scanlines before, you were probably too busy playing the games or were playing on a smaller tube than the one you have currently. Larger tubes have more pronounced scanlines as do multi-sync monitors that need the ability to resolve more lines.
Honestly, the left side images look great!

I would be very happy with that. I guess you can learn to love it or do something like scale your image or defocus your monitor to destroy your picture quality a little.
