To begin with, Assad has to be a complete jerk to use chemical weapons in this stage of the war. First he is winning the war against the rebels, second President Trump just announced a few weeks ago that he was going to pull our troops out of Syria. No doubt that a Syrian plane or helicopter dropped the Chemical barrel bomb because they traced the aircraft back to the Syrian base. The only answer to this I can think of is, SABOTAGE. I honestly believe some high ranking traitorous official smuggled a few barrels of chemicals and had them marked the same way as the regular barrel bombs which got put on the aircraft unknown to the pilots or anyone else.
On the other hand it was Assad and Russia’s responsibility that all these chemical weapons were out of the country or at least hidden and guarded safely. Whatever the case, I’m sure all parities will see that it doesn’t happen again.