Yes, it is only an internal peg. The system believes SBD is always worth 1USD, so currently, you would need 2.8 SBD to buy 1 STEEM. But in the open market, both STEEM and SBD are now worth roughly $2.8 USD. :)

The price data for each processed block carries the same weight, but as the Top20 witnesses produce a block roughly every minute, their feeds are more important, so overall it carries a bigger weight.

I don't know the exact inner-workings of the calculations, but it is an average over the last 3 days.