Why does HFM add additional decimal places to results of simple calculations?

If for example I create a simple rule to add two basic input accounts A and B together, where A has a value of 10 and B has a value of -9.99995, why does HFM store a result of 0.0000499999999998835 instead of simply 0.00005?  This has caused some issues where results can fall just outside a tolerance so can fail a validation test when they should have passed and vice versa.  Would using ROUND in the rules compensate for this?

Many thanks for any help.