I can see how Siri might have gotten a little confused with this one. However, the other day I had a similar issue. I said "Remind me at 10am tomorrow to do laundry." And she responded with "OK, I'll remind you tomorrow at 9am to do laundry at 10am." I have no idea where she got the 9am from. And yes, I do need to remind myself to do laundry.
But can someone tell me why my statement was not understood?