I don't know -- maybe engagement rings are more significant to women...maybe just because traditionally women get a ring before the wedding ceremony. (There is a thread about why guys do NOT get a pre-ceremony ring -- that's worth pondering, too.)
To me, the important thing here is that he went out and bought it for you EVEN THOUGH he didn't understand it. That is, there was a level that he knew it was important FOR YOU, and he was quite happy to just make you happy...EVEN THOUGH he was somewhat confused as to why that particular thing would make you happy.
My guy does many things for me, or with me in mind, that he doesn't totally "get" what I get out of it. I adore him for his willingness to do that. I admire his ability to just accept my 'stuff' that is important to me, without having to understand every minute detail of it before he can act in ways that make me feel special.
Just my 2 cents but...I would wear that ring with pride and joy and pleasure

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Congrats on your engagement!
EDIT TO ADD: Not that he couldn't have voiced his confusion in a bit 'nicer' manner, mind you -- call it a mouth malfunction on his part, and you guys will have something to laugh about on your 10th wedding anniversary.