Eeeesh. Hard to tell.
Sometimes it is due to some gaps in knowledge/certain areas and The conomist test may have gone in that direction. It is not that hard to mess up a test actually - I have been playing with CAT algorithm for a few months now with the new
GMAT Club tests and I can tell you first that, a few wrong questions here and there will make a big difference with the adaptive system. (they are smart but also very judgmental). Final reason I can think of - seeing repeat questions. Did you have any in your Kaplan tests?
I would not take it too hard to your heart and would average for now - 660. You can take other tests and see where you fit in with greater accuracy. However, if you are not quite done with your prep, there is not a ton of reason to keep taking CAT's - they will only mislead you.