Sending messages: get delivered message except for one friend (same phone) it says "read" anyone see this?

I'm curious about something that I hope someone can clear up. I have an IPhone 5 iOS 6.1.4. I send a lot of messages and always get the "delivered" note after

I send them. However, with one person-just one-who has the same phone the little note says "read" instead of "delivered" on my end only.

She doesn't see it, her note is as usual. Obviously, this is not a problem but curious mind that I have it bugs me. Has anyone else seen this

and can anyone tell me what it's happening? Thanks!!

Kimmons