Currency Rounding

This is the first time I am actually using foreign currency so I am not actually familiar with its possibilities and limitations.

I created a new currency and set the rounding to 100.
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Is there a reason why its not rounding it to the nearest 100? Basically KHR doesn’t have cash notes below 100. So obviously you can settle the 26,800 but not the 45.

How can I make this round properly? Whats this Rounding field used for?