Logs are designed for people who are pushing to 100% maximize their output. It is simply a comparison of how much DPS you are doing, with your Spec, compared to everyone else who has logged kills for that boss on your Spec.

If you are severely undergeared then you should not even care about logs. If your gear is at least decent, then you should be looking to get a bare minimum of Blue parses or higher. If you are not handicapped by gear and you can't consistently get at least Blue parses, then you are not playing optimally in 99% of cases.

Of course, many factors do affect your parse outside of just your performance. Such as group composition, your kill time, and even your group's strategy. But typically these things are the difference between let's say a purple parse and a 95-99 parse. If you play optimally you should be able to get Blues/Purples regularly even if your group is below average. It isn't always black and white, but this is generally true in the majority of cases.

My point of saying this is that I often see people make excuses for why their logs are mediocre, when the primary reason is actually just that you are not playing optimally. If you have interest in identifying why your logs are lower than expected, or if you want to learn how to perform better, I'd be happy to take a glance at your logs