How did this happen with a sim card ?

I'm curious to know how this could have happened. My daughters Meteor phone was broken and she asked me for a lend of my phone which is vodafone. She removed my sim card and replaced it with her own. She sent some text messages, two of which needed replies. She then removed her sim card and I put my own back. When I then switched on my phone two messages came in for her ( the two replies she was expecting) How can that have happened, as it was my sim card in the phone then ?
There probably is a simple explanation, but just cant see it. I would be grateful if anyone can explain.

The phone number is tied to the service provider's sim-card, not the phone. I'm surprised it worked.

Im guessing here but possibly they came in and went onto your phones memory at the precise moment she was switching it off so the 'new message' alert didnt get a chance to display til it was switched on again.