Rich Client number format auto rounding issue

I am using the Web Intelligence Rich Client 4.2 SP3 Patch 3. There is a column that I set the number format to be "####.00000000000000". However, when I extract data from my sql database, if there is '99' in the decimal place, it round up automatically.

For exmaple:

3.456123456456 is normal, returns 3.45612345645600

but 30.059989 becomes 30.06000000000000

I cannot change the column type to String/Character because I need to do some summation on it. Any ideas why it happens and how to fix it? Thanks a lot!