Yeah it's the only explanation. Because it's even worse when you see them ingame. They should offer nearly no protection, because the coverage is barely better than that of peasant hats. Secondly, I think the mail coif would need a buff if we considered its historical effectiveness.
Remember that almost every single helmet has a mail coif. It shouldn't give better protection than the helmets with mail coifs and additional stuff, but I do agree it should give WAY more than a bloody cap. Even if the cap has metal in it!
EDIT:
Just found this for the lulz.
Seems like a turban doubles the protection a mail coif gives xD
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loginSarranid Mail Coif
weight: 2.1
head armor: 41
difficulty: 11
And the Kettle Helmet (a helmet with a mail coif beneath) gives LESS protection than the mail coif with a turban xD
visitors can't see pics , please
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loginKettle Helmet
weight: 1.6
head armor: 36
difficulty: 10
This shit is just so wrong...