It looks like blockchain.info is using different conversion rates in different places. Why they are doing this (and where they are getting their data) is really something only the owners know.
I'd like to thank you for bringing this to our attention; however as blockchain.info is an important source of data for the entire community and it's important that we are aware of discrepancies like this.
As for the second part, the answer lies in the field of economics. The truth is the price of things -- that is, what people are willing to give up for something else -- is constantly changing throughout the world, though you may not notice it locally, especially if you are used to a fixed pricing system as is used in most of the western world. In a way, our financial systems are built to mask this fluctuation, but bitcoin is new and our financials systems simply aren't built to cope with it, thus we are currently seeing a lot of unusual market phenomena involving it.
Bitcoin, like all things, actually, doesn't really have a "fixed price" -- just how much, on average a bunch of people are willing to sell it for. The price that you see on the web is actually based on an average of what many people have posted on many exchanges as their own individual bids that they are willing to share and for how much. Different sites read different bids and possibly use different methods of calculating this average, so this is why you see varying prices in various places.
It should also be noted that a person may sell different amounts of bitcoin for different prices, say 10 BTC for $100 and 20 BTC for $180 (A 10% discount for buying double!). This can also affect changes in price.
Now, since bitcoin's price isn't fixed, you kind of have to throw the idea of "overcharging" out the window. Let's say you buy a bitcoin for $100 and I buy one for $90. Were you overcharged by $10? Perhaps. Or perhaps tomorrow someone may offer you $150 for that bitcoin. It looks like you may have been undercharged by $50. Again, since there is no real fixed price, and what people are charging for it varies so much, the idea of "overcharging" doesn't really make sense. However...
My best advice to avoid "overcharging" is just to be diligent search around for what ever is cheapest, but also be aware of what's most convenient for you.
Coinbase.com is fairly cheap, but it currently it takes about a month before you get full access to instantly purchase bitcoin. There are faster methods, but they are usually on the expensive side and I would generally avoid them after spending you month on Coinbase.
MtGox.com (as I've been informed) suffers from having both a high price and along wait to get money out of it, although their high sell price is appealing to some, such they are willing to forgo the inconvenience.
LocalBitcoins.com and Satoshi Squares (tf you can find or start one) are also good options to meet up and trade bitcons. Here, you may actually be able to negotiate face to face to ensure that you're getting a good price.
