The disparity makes sense if you think about it for a second.
A mail coif by itself is much, much better than having nothing on your head, but you are most likely going to be incapacitated or dead if you get hit in the head with a warhammer, bec, halberd etc. It is at a disadvantage against armor piercing weapons even if it's riveted mail rather than butted mail and offers literally no protection against concussive force.
A mail coif augmented by a metal skullcap and orle, which is what the Sarranid coif is, is a markedly better (although far from perfect) defense against armor piercing weapons/concussive force and will greatly increase your chances of survival over just the coif.
If anything should be changed, it's the name of the Sarranid coif. The stats are actually reasonably reflective of the relative protection offered by each helmet and should probably remain as they are now.