If you choose "Transaction Prices" on the Gains page (cost per unit column) these prices are sourced from Coinmarketcap (weighted average).
This is why they differ from the real prices you paid, as you can check for every trade on the Trade Price report.
You can adjust this by selecting "Best prices" at the drop-down filter on the Gains page. Afterward you can see the real price you have paid on that report equal to the trade price list.
