The net amounts shown at the top are aggregated sums of all your FX trades settling in each currency in the selected period. A positive net amount means that the impact of your FX trades on your account will be positive; a negative net amount means that the impact will be negative.
The volume shown below each net amount is the absolute value of all the underlying traded amounts, both positive and negative. This means that if you both buy and sell a currency, the volume will be different from the net amount.
You can drill down to view the underlying trades in a specific currency by clicking a net amount.