As the title says.
Most folks aren’t metallurgists, and the descriptions are sort of an everyman’s take on the items.
In other words, if the average Joe doesn’t know the difference between ferrite and austenite then the item’s description isn’t likely to differentiate either.
Why not call it a metal blade, then? Calling it iron is misinformation.
Because ‘metal’ covers such a wide range of materials? Aluminum is a metal, and so is magnesium, lead, silver, cobalt, tungsten, and mercury.
Again, most folks aren’t particularly solid on what constitutes the molecular differences between iron and steel, so the character sees it and thinks ‘iron’, while the player can easily read in its materials that it’s made of steel. This gives us the best of both worlds; the in-character description as well as the out-of-character information contained in the item’s information block.
Also, iron and steel are more or less the same in the cataclysm. You won’t be choosing the things because of their material, you will be thankful if you even have one.
This is a bug, most likely caused when we split iron and steel into different materials. I’ve reported it on github, so it should hopefully be fixed soon.
Thanks for reporting this! 
Huzzah!