"Because I have the master list of which frames I have dropped for each recipient and can add back in any you have dropped which aren't on the list. Unless you magically coincide with the same frames by happy accident (and you have to win that lottery multiple times to really obscure the signature) then I can tell the difference between which frames you have dropped and which ones I have dropped."
And should I re-encode to a different frame rate, all is lost.
My point is that once someone knows how you encode something, they can mess it up. They key is to do a range of things and not tell people what you did. That way the only way they can tell is by comparing two leaked copies looking for differences. The odds of getting two leaked copies before the release would be almost impossible.
If done this way the leaker would be terrified that they missed a fingerprint which would incriminate them.