I did exactly that just now. First test gave me .08. I emptied out the vial and cleaned it and repeated the test. The second one gave me .01. I indexed the vial and inserted it exactly the same way each and every time.I think part of the issue may be the simple fact that you aren't following the manufacturer's directions.
You aren't supposed to be using two different vials for the test. Any difference in clarity between the two vials is going to skew the results. Not to mention Hanna vials are known to have different fill to lines. I know mine do....
you should be running this test with a single vial, and you should be indexing it when inserting it into the checker, so that the checker is looking at the same spot in the same vial for both parts of the test.
I bet if you actually run the test as intended, back to back to back, the results would be more consistent.