Because we don't know he took twice the allowance. As an example, a kidney infection can prevent salbutamol passing into the urine and it builds up until the kidney functions properly again, then you can piss out 2 or 3 days of salbutamol in one go.
It's not even a TUE to use an inhaler.
It would seem pretty straight forward to be honest; if you're tested for 21 consecutive days (just in that race, let alone all the days he led Le Tour) where all other days were below the testing level, and one day was inexplicably high, then you have an issue with the test results because if he...
Ulissi was slightly different, he wasn't actually banned for a doping violation and his results actually stood for that period because they found and upheld he had no performance gain from it, he was suspended basically due to his own negligence of not adhering to protocols or the prescription...
It's perhaps not fair on Froome, or Wiggins for that matter but Sky had it coming through being so arrogant as to say they were going to be zero tolerant and set standards above and beyond all other cycling teams, then in that heightened environment with everyone on tenderhooks about doping...