I find that each sensor has a range, and they can vary quite a bit. So it is possible (although I am not making any assumptions) that the difference between your first and sensor readings was the sensor, rather than a significant difference in your actual blood glucose levels.
For instance, most of my sensors show readings in the 5-6 range, but then occasionally one will turn up that sits in the 6-7 range, even though there is no significant change in anything else. The next sensor will then read as normal (5s and 6s). There was one that measured everything 1.5 mmol/l higher than 'normal' which I verified against prick tests. Didn't bother me at all once I had realised what was happening!
I just think that either the sensors vary slightly, or the position on my arm varies - maybe blood supply, or fat distribution, or something.