I'm sure deception was never the doctor's intention.
It may very well have been an IQ test you were given, but I suspect any kind of written test would have served.
The results of the test were not at issue. IQ doesn't necessarily have anything to do with ADD.
Has it occured to you at all to consider what some of the symptoms or indicators of ADD are? Your doctor was probably observing your body language, as well as your ability (or inability) to concentrate, among other things.
Most people would be very self-conscious if they were told up front that they were being observed and therefore would probably act differently, aware that any move or gesture they made was being analyzed. And the results could have been compromised, possibly even causing a misdiagnosis.