Why is 15.4% taken from my deposit interest?

In addition to the 14% income tax withholding rate on interest and dividend income, personal local income tax equal to 10% of the income tax amount (1.4%) is specially collected at the same time.

Withholding agents are required to collect local income tax at the same time as they withhold income tax. That is why ordinary interest and dividend income is paid with 15.4% deducted.