Light Planchet Errors?

I have approximately 6 each of 1972 and 1979 Kennedy half dollars all weighing 10.4 -10.5 grams. The normal weight for the planchet is 11.3 grams. They are not excessively worn in fact some are AU. I cherrypick Kennedy half dollars for a few varieties and errors and of course silver from time to time. I find these coins by the sound they make when they contact another coin. They resonate with a higher pitch than normal. In fact, they sound like a silver coin would when you plink it against a clad coin. Just from the sound of the coins in a stack or roll, I can tell that one is different. I can pick out the exact coin and sure enough when I weigh that one, it comes out grossly underweight. I have asked some experts in the past and they have given me possible explanations like (foreign planchet?) however I have looked up the foreign planchets minted during those years as published by the US mint and can't find any that match the weight of the coins. Other experts have suggested that the coin planchet was just rolled a bit thin. I can't tell any difference in the thickness and they all appear to be normal thickness. Further I had two of the coins tested and certified by SEGS and the grader told me that he used a micrometer to measure the thickness of the coin and it was within the accepted standards for half dollars of those years. Further, the coins all appear normally struck, I don't see any weak design features that aren't common to all kennedys of those years. I've been frustrated by my inability to figure this one out. SEGS marked the coins as light planchet when they encapsulated them ( of course that doesn't tell me anything more than what my scale already told me) SEGS also said they did a specific gravity test that came back normal for Kennedy halves. Something doesn't add up. If the outside dimensions of the coin are right and the specific gravity is right, then how can they be underweight? Is it possible that there were experimental planchets used in those years? Is it possible that I don't have a comprehensive list of foreign planchets possible for those years? Any help would be appreciated. I don't want to spend lots of money seeking an answer through certification (I've already spent some without any answers) and still come up with nothing of value. Thanks for reading this long post. I can answer further questions if you like. The fact that these coins are all 72 p or 79 p and no other years keeps leading me to believe that something explainable and not random was happening in those years in Philadelphia!