Why is the sum of the rates over all counties in a HSE area different to the rates of that health board as supplied by the website?

The rates are pre-calculated to 14 decimal places at the registry, rounded down to two decimal places and then uploaded to the website.

When you add the rates for all the counties you are dealing with figures which have been rounded to two decimal places. Because of this, a rounding error creeps in, which is not present in the rates supplied by the website which were calculated to 14 decimal places and then rounded down.