I guess the data type is Double? Decimal might require you to set the Scale property to get expected results, but the common advice is to avoid decimal type anyway. Depending on the underlying data and desired level of precision, perhaps your query only needs a format for the desired number of places to the right of the decimal:
Result: Format([mynumber1]-[mynumber2],"0.00000")
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.