Why has a lower than expected tax percentage has been withheld on a tax payment?

Most likely this has to do with securities lending. It may be that (a part of) your position was lent during the dividend payment. In that case, you do not receive a dividend, but rather a dividend substitute payment.

In the WebTrader no distinction is made between a dividend and dividend substitute payment; both are categorized under the description of dividend. A dividend substitute payment is generally not subject to withholding tax. As a result, the total withholding tax paid as a percentage of the dividend presented in the WebTrader can be lower than the statutory rate of the source country.