If you go to http://backpack.tf/pricelist?category=all and search for the Hetman's Headpiece and it says unique is 1.33 ref but uncraftable is 18 KEYS! I thought uncraftable made it less valuable. How does that work? Are there any other hats with this exception?
The Hetman's Headpiece is a promotional item that's awarded in Genuine quality to players who pre-ordered Mount & Blade: With Fire & Sword. Later it would be made available in the Unique variety via crafting and the item drop system; however these are always craftable (aka clean).
Uncraftable (aka dirty) promotional items such as the Hetman's Headpiece are not supposed to exist, but technically can still be obtained either through glitches or some other obscure methods. Therefore very few dirty Hetman's Headpieces (and certain other promos) actually exist and as such, any of these that do exist are extremely rare and valuable, at least to collectors. According to stats.tf, as of this writing only 27 uncraftable Hetman's Headpieces exist out of a total of almost 30,000 of this item in total. That's less than 0.1%.
If an item is widely available as an uncraftable item, then it will very likely be less valuable than its craftable counterpart.