I think you're trying to make too much sense of it and go by a organized structure we're used to now from a time where everything was pretty much chaos and disorganized. But Gary was ranked top 10 on some sites.
About him being 17-13, I don't think you can really judge a fighter based off his entire record when talking ranking, I think you gotta narrow it down to a more specific time frame. As mentioned above, Gary had the best win/loss record between 2000 and 2005 when he fought Fedor. So what happened prior to after isn't really relevant. I mean you know there are dozens and dozens of cases where a fighter has a average record and then at a certain point in their career they get it together. Like Jorge Masvidal for example, the man was 32-13 before he even became a top level fighter. RDA for example, started his career 15-6, then mid career when he had his best run went 11-3, and then since then he's 2-4. Not saying Gary and these two are on the same level, just that fighter hit their peak and rankings at a certain point and then the rest of their careers they're lesser fighters.
Fedor had a 27 win streak but its a very unique scenario. You can't really compare him to a standard mma streak. First, he won 27 fights in a row which is rare, it was between like 5-6 organizations, contract disputes etc. It's an odd scenario. He's bound to have inconsistency in competition.