Why is the sum of incidence/mortality rates such as C00, C01, ..., C15 different to the totals (e.g. C00-C14) 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 tumours yourself, or using the websites "All Cancers Combined" facility, 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 total rates supplied by the website which were originally calculated to 14 decimal places before being rounded down.