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.