Why have my funds in Bitcoin Block Explorer's online wallet decreased in value? I feel I may have been cheated by Bitcoin Block Explorer whose URL is http://blockchain.info

Here's the background:

I am using the online wallet provided by Bitcoin Block Explorer. It contains 1.5 BTC.

According to the website http://blockchain.info/stats, the weighted average price is 134.19 USD for 1 BTC on September 11. It means that my 1.5 BTC  is worth 201.29 USD.

However after logging in to my wallet, on the top right-hand corner, the USD equivalent shows 186.18 USD. It means the weighted average price is 124.12 USD.

Could some experienced guy explain to me the discrepancy?

Added a second question

After going through the comments posted here, a second question comes to mind.

An online merchant gives his customers the option of paying using one of the following methods: Visa, PayPal or Bitcoins.

The price of the product is 50 Euros if the buyer uses Visa or PayPal to pay but 0.52 BTC if he chooses the Bitcoin payment system.

According to some posters here, Bitcoin/USD rates vary across exchanges.

My question is: if I am the customer, how can I be sure that I am not overcharged if I use Bitcoins to pay? (0.52 BTC may worth more than 50 Euros)
Bitcoins are traded on many different exchanges.  The prices are varying  constantly just like any currency.  As an example:  at this time three rates are Mt.Gox - $143.0983; CampBX - $125.8000; and BTC-E - $124.0000 (all USD).  As I learn more, and become familiar with the exchanges, I may try some arbitrage.  I have a feeling there are problems, or people would be doing it now, and all the exchanges would be more in line with each other.