XPath in XQuery

Using the following output XSD type:

<xs:simpleType name="MoneyType">
  <xs:restriction base="xs:decimal">
       <xs:fractionDigits value="2"/>
   </xs:restriction>
</xs:simpleType>

I use a fn:sum command and get the following output:

1.0001865499999999E6

The value should be

1000186.55

What would cause this lack of precision

Thanks