It is also possible that whatever service you used to buy BTC is using a different data source for it's BTC to $ conversion than your wallet, which would explain some discrepancy... not sure it is likely to be responsible for 50% tho...
I would think that Abdussamad is likely close to the mark... the service you used probably has horrific exchange rates and fees... what service did you use? and are their rates and fees listed clearly?