On 23 June 2014, the last declared chemical weapons were shipped out of Syria for destruction. The destruction of the most dangerous chemical weapons was performed at sea aboard the Cape Ray, a vessel of the United States Maritime Administration's Ready Reserve Force, crewed with U.S. civilian merchant mariners. The actual destruction operations, performed by a team of U.S. Army civilians and contractors, destroyed 600 metric tons of chemical agents in 42 days.
So, basically, assad gave over his chemical weapons stockpile to the usa. However, it appears that the usa may not have destroyed them, and instead is using them against syria. I heard on 60 minutes last night that the proof the usa has that assad used chemical weapons is because the tests from the chemical weapons stockpile matches the chemical weapons residue found at the locations of the attack(s).
It was either a mistake to hand over the chemical weapons to the usa because it gives them the argument that because they have the chemical weapons, they can say they match the attack residues. If syria had not given the weapons to the usa, and instead to a friendly nation for destruction, the usa would not be able to argue that the residue matches, because they would not have the stockpile to compare against. This would be the case for attacks that happened before the hand over of the stockpile to the usa.
For attacks that happened after the handover of the stockpile, and I do not know the details, but I think the type of gas varied, the usa not only has the argument I stated in the previous paragraph, but there is also the possibility all syrias weapons were not destroyed, and were instead directly handed over to anti assad forces whomever they may be(isis, israel, US army, turkey, I do not know).
Does this make sense? I think it is possible.