I think the 10% sarin mortality rate was from those making hospital alive.
Very many were killed and didn't make hospital.
Highly graphic *WARNING* video of some that didn't make hospital:
https://www.youtube.com/watch?v=W2r7hUwAxxc
Photographs and analysis of the actual chemical weapons used:
Preliminary analysis of alleged CW munitions used in Syria | The Rogue Adventurer