I never actually understood the BTC rates on preev
BTC rates on Preev aren't taken from thin air. They display rates taken from Bitfinex, Bitstamp, Kraken and LocalBitclins. I don't know why Preev become a standard when checking BTC prices, but probably it's mainly because it was simpliest and most convenient place to check it when there wasn't so many options as we have today.
There is a gear icon at the top-right corner of the page that shows where the rates come from and which exchanges you want to consider/disconsider.
By default, it uses the medium rates between Bitstamp and Kraken, which I believe makes sense to price BTC trades. People could just use the exchange which the biggest/lowest price (based on if they are selling or buying) to get the best rate (for them). This way it's good for both sides.
There is also a note about this in the bottom of the page:
Price data is continually gathered from multiple markets. A weighted average price of these markets is shown by default (based on 24-hour trade volume). Alternatively, you can choose a specific source from the settings menu.