For instance, a DLC pack with a single positive review appears above a major game with a 74% score and 15,000+ ratings.
The current "user rating" ranking system seems to divide everything into big semantic buckets ("Overwhelmingly Positive","Positive","Mixed",etc), stack those in order, then sort each bucket's contents by the total number of reviews per game. Given that Steam reviews skew massively positive, ( half are "very positive" or higher), this is virtually indistinguishable from a standard "most popular" chart.
Luckily, there's a known solution to this problem — use statistical sampling to account for disparate numbers of user reviews, which gives "hidden gems" with statistically significant high positive ratings, but less popularity, a fighting chance against games that are already dominating the charts.
More details: Fixing Steam's User Rating Charts