Why is my profit on SCB lower than it is supposed to be? Is there any fees on the profit? I use an external app to track my profit and it doesn’t tally with SCB?

It's usually because they take into account the commission fees

Yes, it will be lower. SCB includes the buying/selling transaction costs in the profit calculation. For example if you bought a stock and your transaction fee is SGD10+, your initial Profit shown will be -SGD20+ (buy and sell costs). You can try to do a another calculation and check.