This may happen once your portfolio is invested and the actual weights of your ETFs have moved away from their target weights.
Your Fund Portfolio order will seek to realign your portfolio to its target weights, investing in the most underweight ETFs first.
But how many ETFs in your portfolio are topped up by your order will depend on how far your ETFs have moved away from their target weights as well as the total amount of cash being invested.
Similarly, some of your ETFs may not be traded in a Sell Portfolio order.
With this type of order, the most overweight ETFs are sold first, and how many of your holdings are traded will depend on how far your portfolio is from its target weights and the cash proceeds you want.
An additional issue is that if a holding has yet to settle following a previous trade, it cannot be traded in a Sell Portfolio order — and is excluded from the calculation of how much of each ETF to sell.