It happens in some simplified models. Suppose a thousand tracks get released every year, and these tracks are evenly divided among 50 different types of music, 20 tracks per type. Some of these types of music are very popular (4.95 stars), some are slightly less popular (4.90 stars, maybe because they offend some powerful group), and some are much less popular (4.00 stars). So the most popular 100 tracks will be the 20 tracks from each of the 5 most popular music types.

If we increase the number of tracks to ten thousand, there are a couple of ways we can go. We could increase the number of types of music to 500, and in that case, we'd see better music rising to the top. (Or at least more popular music, which may or may not coincide with being better.) Or we could increase the number of tracks per type to 200, in which case a random half of the tracks of the most popular 4.95-star music type will be the "top 100" for the year.† Or we could go for the middle of the road: maybe now we have 150 types of music and 67 tracks of each one. Or we could have musicians and record companies that respond to incentives by trying to produce more music of the popular types, somewhat handicapped by the fact that those popular types change every year, and however much Nickelback might try it, recording the same song under twenty different titles doesn't actually give you twenty top-ten hits.

Regardless, there are a lot of ways that more published music could both provide more variety and less varied top hits.

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†Of course the Billboard Hot 100 is per week, not per year, but that's the least of the oversimplifications in this model!