I was wondering about this too - the exported excel sheet from the exchange contains all prices (if some coin was traded for USD), so I'm not sure why some averaged coin price across multiple exchanges is used rather than the price in the spreadsheet.
If some coins were traded for USD, CoinTracking will use the USD price for the calculations.
But if a coin was traded for another coin, we need to calculate the value based on average historical prices.
Each exchange has it's own prices and there is no global price for coins like for fiat currencies.
That's why we use a weighted average.
But I will work on adding additional prices for different exchanges.
Dario