The mithril armour values are messed up. For example, the chain mail provides more protection than the chestplate. 

[Mithril Chestplate]
[Mithril Chain Mail]

Steel armour values are funky as well. Chestplate is not a real improvement over chain mail and the steel kite shield is worse than the normal shield.

Not sure, they look decent to me. Normal shields will always have higher armor, but less block than Kite shields. Same thing goes with Chestplate / Chain Mail and Helm / Large Helm. Just because one is higher to craft, doesn't exactly mean that it is "better". It all depends on how you want to build your person.
Someone going for max damage will always go Chainmail and Helmet, where someone who wants to be more tanky will go for Platemail and Large Helm.