When you have a big difference in your tax gains between the different methods (Best price, counterpart, transactioin price) you very likely have traded a rare coin or ICO coin,where the price changed very quickly within a day. Thus depending on whether the value of the big coin (eg BTC or ETH) is used or of the small coins, the result differs a lot.
You can view this on the Trade Prices page: If the spread is too high (more than 5-10 percent), then there will be different results for different calculations. Please check,if 1) the time of your trades are correct, the time zone is set correctly and if the coins are correct.
If this does not correct the issue maybe our price is wrong or not exact because CoinMarketCap has sent a wrong price. In that case you can change the asset value manually on the Transa page.