Naomi,

Here's an example to show you what is happening.

with 2 decimals

     9.99
    10.23
    10.55
sum 30.77

with currency

     9.9999
    10.2390
    10.5590
sum 30.7970

shift to 2 decimals 30.80

Now extend that to a larger number of values and you can see where the $5 discrepancy comes from.