When you eat your body uses the food for different actions in your body. Any fat could be recorded in your blood and give a false reading. Smoking before can also give a higher reading.
How does this work then .... My sister had a test and forgot not to eat, so she had breakfast. Her result was 5.8. Feeling this was too high, she had a repeat the next week after 12hr fasting. The result was 6.!!!!
As well as absorbing cholesteror from food cholesterol is manufactured in the body ( I think in the liver but could be wrong) Diet plays a part in lowering cholesterol but excercise and drugs may also be needed to control this.
You dont to get a cholesterol reading - but it does affect triglycerides - another component of fats in our blood. You may need to starve for a blood glucose which is often done at the same time in some problems such as blood pressure or heart disease to see if there is diabetes. Cholesterol varies as do all biological values as does the test that it is used. So 5.8 one day 6.0 another - it does however vary according to the seasons!