I did say it was a guess as to why they did it.
Generally when there are multiple opposing claims you can get to the truth by comparing the stories, I.E:
From what's been claimed/admitted we know that the airstrike was either carried out by the Syrian government, the Russians or the west (the rebels have no airforce and ISIS planes don't work). The Russians and the west are both saying it was the Syrian government and the Syrian government admit it so that's pretty conclusive that it was the Syrian government who carried out the airstrike.
Now, from the pictures/videos/eyewitness testimony it's pretty apparent that chemical weapons were involved, the type isn't yet known (though Turkey are claiming Sarin based on autopsies they managed in record time, dubious) but Russia and the Syrian government are claiming it was a rebel chemical weapons dump that was bombed and released, the west and the rebels are claiming it was the airstrike that deployed the chemical weapons, and ISIS haven't released a statement.
This really leaves the question of which of these two scenarios is more plausible, either A: That the gas came from bombs dropped by planes belonging to a side that had chemical weapons and had allegedly used them before. Or B: that regular bombs landed on a weapons dump and set off some advanced chemical weapons which wouldn't go off if bombed.
I am of the opinion that It was the Syrian government who used the weapons (which may shock some posters as I usually get labelled an Assad apologist for finding the extremists winning less desirable) because that's simply what makes more sense and is more plausible.