Let’s be real here, AR probably forgot how she had described it or hadn’t decided on how it should be, bc IIRC it’s only those two instances in all of canon that it’s described as straight, and those were in books 1 and 3, while she might still have been developing him as a character. 

 IIRC, Armand’s hair is described as curly when he was mortal and when he was a fledgling, so I think that’s the natural state of it. I don’t recall him ever mentioning straightening it, but when it’s mentioned in IWTV, it’s the 1970′s, and in QOTD, it’s the 1980′s, so he had access to straightening products if he felt like straightening his hair.

^X @amadeo-child-of-the-renaissance found this, another Botticelli angel Armand 😉 AR bases Armand off these Botticelli angels and they have varying straight and curly hair.