I do not know if they are or are not the same turtles, but incorrectly measuring once can conceivably go with incorrectly measuring twice.
If I came into possession of a species I am not familiar with, there would be a learning curve. To be in the reptile business as Andrew is does not mean he knows a lot about all reptiles or the conventions for measuring them. I am not by any means giving him a pass on inaccurate measurements or what have you, but these errors are at least conceivable if he is much more a dedicated squamate guy and much less a dedicated chelonian guy.
I am not saying you are responsible for your loss (depending on what comprises that or defines it). The size discrepancy was sort of accepted according to the text and the approximate descriptor gives him some wiggle room, too. The legitimacy of the labeling for the genetics (as het for caramel) is something I cannot even begin to know since it will take test breeding to prove out. You have at least three very knowledgeable guys who are said to disbelieve the label and I can understand and recognize that. The problem with that piece of things is that they do not know everyone, what everyone has, or from whom they got what they have. I know people with breeding projects of caramel/snow sliders that have never chosen to advertise and their hets are as real as anyone's. They can provide chain of custody and lineage information if they want to, but if they did not for some reason (privacy or maybe protecting a source), the turtles would be just the same in terms of genetics. It would have been in Andrew's best interest to disclose lineage/whatever information before the transaction was done. Back in the "courting" phase of the deal, I mean. It also would have been in your best interest to demand that information prior to payment. What is done is done now, though. If Andrew is being pursued through legal means over the matter, he might want to wait to share that information (if he has it; he might or might not have it). This is an unfortunate situation at this point and I am not sure how it will go, but the case is not an easy one to win. That is not said to discourage you, but there was a sort of flip-flopping of terms at points in the deal and that complicates things a lot. I hope for your sake that, when they mature enough and add a little more size, they prove out. If they do not, then there is the matter of the offspring value and splitting that out if the transaction is reversed or not if kept and used to recoup the value difference in the sale. Either way, you will at least be out time if they do not prove out (which is not technically the same as disproving, unfortunately). No fun ahead in my opinion.