title: Java units of measurement library addition and subtraction returning incorrect values
body: <p>I'm using the reference implementation of JSR363. I've tried many variations of this but I'll give this code as an example.</p>
  
  <pre><code>ServiceProvider provider = ServiceProvider.current();
  QuantityFactory&lt;Length&gt; lengthFactory = provider.getQuantityFactory(Length.class);
  Quantity&lt;Length&gt; first = lengthFactory.create(5, Units.METRE.divide(100.0));
  Quantity&lt;Length&gt; second = lengthFactory.create(3, Units.METRE);
  System.out.println(second.add(first));
  </code></pre>
  
  <p>This prints 503.0 m. Clearly something is very wrong and this should be 3.05 m. I find it very hard to believe that this is actually a bug with the library and I'm hoping someone can tell me what I'm missing.</p>