This type of problem is very common and is almost always caused by the way your different computer systems round numbers. 

VAT calculation in the UK is a good example of this. There are two popular ways of calculating VAT, one that applies it to each line and the other that applies it to the total. Both are logical and valid, but when compared, can have slightly different results.

Sage, for example, calculates VAT by applying it to the total for each line, rounding the lines individually and then adding these line totals together to reach the grand total.

Examples:

You are selling 1 banana at £11.23 each and 1 apple at £9.35

Calculated in line

Calculated at end

Both of the above examples make logical sense as a method for calculating the order total and are both commonplace within computer systems BUT as you can see both have arrived at a different answer!

OpenCRM has a few options for calculating VAT, to find out more please refer to this FAQ: https://opencrm.co.uk/help/?kb=705488&article=How-does-OpenCRM-calculate-VAT-compared-to-Sage