Total in SGD is the sum of the Current Cost of all the different currencies and FX gain/loss.
Therefore, it is possible, for example, for the Total in SGD to be lower than the Total in USD if there is an FX loss for USD.
An extreme (but possible) example:
You bought a stock at USD1 when USD to SGD is at 2.
Total cost in USD = 1
Total cost in SGD = 2
The stock is now worth USD1.5 but USD to SGD has weakened to 1.
Total value in USD now = 1.5
Total Value in SGD now = 1.5
P&L in USD = 1.5 - 1 = 0.5, but
P&L in SGD = 1.5 - 2 = -0.5