is it the testing regime or what happens with the samples taken afterwards ?
as I remember back in 2012 chatting to a USADA doctor/tester on a tube train to Stratford, as you do, terribly unBritish of me I know, but its the craziness of being in a host city for an Olympics. But the two things he was confident on were there were almost certainly people doping at the Olympics, (which he was right about) even though that seemed a shocking thing to say, but that they would also be caught...maybe not that day but eventually they would (which he was also right about).
and by that he meant how they might not have the tests or accuracy to test in the moment, and an athlete might pass and all seem well, but they store the samples for a number of years, it was 8 years then, its now 10 years.
So 10 years from now they can go back and test all the samples from Paris again, with technology and science thats 10 years advanced from now, and if it picks up anything that means you actually fail, you maintain the same legal consequence of failing as if youd failed the day of the competition.
I dont know if TdF store 10 years worth of drug tests, just in case, or theyre destroyed once theyve all been checked and passed