Understanding Bitcoin prices I followed the prices of Bitcoins during this day, but I am actually very cofused as to why Preev reports it always about $70-$100 less than Bitcoinity. News websites say the price soared to $900 today, which fits Bitcoinity's chart, but I also see people on the BitcoinTalk forums saying it is $500 now, which fits Preev's.

I understand Bitcoinity says that "the big number here is the price at which last trade was made". But since I also understand there is no institution that decides an "exchange rate" for Bitcoins, could I assume it as the price of the last trades? Why such a big difference?

EDIT: Now Preev is reporting more, but it is still a big difference compared to Bitcoinity. Just as a reference, when I wrote this question, Preev was reporting about $510, whereas Bitcoinity $600.
Different exchanges will offer different rates at the same time because of variations in trading volume, markets, etc. This happens in real currency markets, as well. With enough money it is even possible to play the currency market, "buying" dollars with Euros, then buying yen with dollars, then buying euros with yen, and coming out ahead.