Why is the top filter lighter than the bottom filter on my match weight cassette, resulting in a negative weight?

Match weighed cassettes have the filters matched within a specified amount (20, 50 or 100µg). If the sample amount collected is less than the difference in weight of the 2 filters, it is possible to have a negative weight if the heavier filter is on the bottom. This is not an error in the product. This would be an indication that either not enough of a sample was taken or it is a very clean environment.