Could be anything
Stress, different activity levels, fatigue, sleep deprivation, different portion size... even just the fact that it was a Saturday, so your routine was different.
Also worth remembering that these home blood glucose meters are only required to be accurate to within 15% plus or minus. So an actual bg level of 10 could show up on your meter as a reading anywhere from 9.5 up to 11.5 and still be considered to be acceptably accurate. You also sometimes get really weird readings (e.g. 15 when you were expecting a 5. In that case it was was probably a dodgy strip, and just re-test).
It is very important when you are doing home testing to remember that you are looking for trends. Never think that any single reading is accurate. And always test a meal, with the same portion sizes, at least 3 times before you are confident that you know how your body reacts. Although stress, tiredness and activity will all still have an impact.
It really is more of an art, than a science.
I really feel for the insulin dependents. They have the same meter inaccuracies, and they have to use the information to calculate their insulin, but they have the same factors affecting things. It is so much more difficult for them.