ARB should be giving you the exact data sent to it. I'd recommend pulling a Data Warehouse report to double check and make sure all decimal places are actually in the data set. If they are, try pulling data requests from a brand new workbook to make sure it's just not Excel giving you the rounding.
Once you've verified both of those and there is a genuine discrepancy here, I'd recommend filing an incident with Customer Care so they can log it as a bug.