Let's assume you bought 1 BTC for 100 Euro and then sold 0.5 BTC for 60 Euro.
You therefore have 0.5 BTC left that you bought for 50 Euro and you made a 10 Euro realized gain and a 1455,37 Euro unrealized gain (assuming the current BTC price is 3010,74 Euro).
Since the cost column in the summary table (the first table at the top) shows only the cost of the REMAINING coins, you would see on the "Realized and Unrealized Gain" Page:
What you would NOT see is the 50 Euro you originally spent on the already sold 0.5 BTC. Therefore the summary table only shows 50 Euro value of coins owned and 10 Euro realized gains.
You would not be able to see the 100 Euro invested - only if you had NOT sold any coins would the Cost reflect the sum invested. If you sold everything, the cost would be 0 (since there are no coins left).
Thus the calculation Original spending for currencies = Currency cost + Realized gain/loss would only be correct if you kept all coins and would otherwise always miss the purchase value of the coins sold already (since only the gain or loss would show up in the last column).
You can however find the value of the original spending for currencies in the Realized Gains table (blue link below the summary table). Here the Cumulated Cost will be displayed in addition to the gain or loss.