Usually one of three reasons, and rarely a system error.
Currency. Most holdings are priced in USD. If the ringgit strengthens against the dollar, your MYR value falls even when the underlying price is flat. It cuts both ways.
Timing. ETF prices you check on external sites update in real time during US market hours. Your portfolio is valued at a specific point, so the two will not match intraday.
Fees and transaction costs are deducted from your balance and show as a small drag, especially on small balances early on.
If the gap is large and persistent across all three explanations, raise it with support with a screenshot.