Most of you already know this, but I though the explanation might help some new users.
The term quality in reference to transcoding of the video is actually a bit misleading. I believe the number in DVDFab refers to the proportional size of the original video to the copied video.
In other words, if the original was a dual layer DVD-9 disk (which most are) that was entirely full, copying it to a single layer DVD-5 disk would result in a quality factor near 50%, because the video size would have been reduced by half.
The actual quality is based on the bit rate of the content on the original DVD, which varies considerably from one DVD to another, based on the amount of content (recording time) of the original. So, although a 2 hour DVD that fills a DVD-9 results in a near 50% quality on a DVD-5, that 50% is much higher than a 4 hour DVD-9 reduced to 50% on a DVD-5.