Messages to another iphone sometimes green, sometimes green with 'sent as text message'

I'm texting another iPhone and think she has her imessage turned off and most of the time hasn't got wifi or data. I have imessage turned on as well as wifi/data. Some of my texts that go through to her, are just green. However, others are green but with 'imessage' text above and then 'sent as text message' below. Why is this? Does this only happen when the recipient has wifi but also has their imessage turned off?