That's just it Matt, testing pre meal, i.e. fasting is usually going to show a low average. It is the all important post meal readings which will bump up that average.
If you were to only go by pre meal levels your average would only tell you what the fasting levels were like. Nothing more. That's what my friends did....... When they started testing post meal times every day for a week that is when one of them had average levels of 18 and the Type 1 in the 20's !!!
It makes a huge difference when you test, the Nurse should know that.
As for testing at different times, at 45 mins to an hour you would normally be at peak levels, a spike. Then you would test at 2 hrs to see if you had dropped back down to pre meal levels. The slight problem is, if you eat a meal containing a high proportion of fat or complex carbohydrates then the absorption of the glucose is slowed down and you may then need to test at 3, 4 or 5 hrs sometimes even longer. What is known as the 'Pizza Effect' The peak would then be much later.
Complicated but best explanation I have for now.