It has to be a bug. How can you wear more than one piece of headgear and have it not be a bug?
Lol, as if that's the biggest issue in Skyrim, you could ask "why" about lots of things related to Bethesda. Yes, it's definitely not intended, and they didn't fix it because they didn't fix a lot of things.
Technically many combinations would actually make sense, such as using all kinds of helms and circlets with hoods. But yeah, this is a bug because we know Bethesda worked hard to remove any kind of variety in the apparel department.
This has been known for long now. People are exploiting that like crazy
As far as I know the Arch-Mage robes are the only hooded robes that you can wear a circlet with.
Normally helms occupy two headslots, hair & circlet. These "bugged" helms use only hair slot, making it possible to use circlet at the same time.