That’s how the system works.
If you have a brand new toon (with no previous MMR), you can queue for Solo Shuffle and see where the current average rating is at in your first placement game. (I’ve never tried, but you might even be able to get this info by queuing as a spec you’ve never played before.)
I’m not familiar with your site. I didn’t see an overall rating distribution thing. I might have missed it.
In your example, I think you’re looking at data for a specific spec, i.e. havoc DH, which just so happens to be one of the strongest DPS specs this season in Solo Shuffle. Havoc DHs’ average rating is probably higher than almost everyone else’s. An average DH still needs to outperform a majority of players (all DPS specs) to hit 1800.