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 buyers pays using his Visa card or PayPal 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 pay using Bitcoins? (0.52 BTC may worth more than 50 Euros)