Could be a lot of things.
Meter variation (they are not 100% accurate and 0.5 could well be the margin of error depending on level of reading)
Stress
Extended Dawn Phenomenon
Exercise
Diabetes being a cussed pain in the butt
Excitement
Don't fret it will probably be something different tomorrow try to concentrate on general trends rather than individual numbers (therein lies testing madness!).
As @bulkbiker said, it could be any of those things, but 0.5 difference is well within tolerance levels of the meter. They are only required to be accurate to within 15%.