I've always fed my kitties twice a day (breakfast and dinner), and used to leave dry out for grazing during the day. I think the "eat before a shot" assumes that's the norm. Since the shots are 12 hours apart, and the meals are too, then the "normal" sequence is to test pre-shot, then feed their canned food, and shoot right after or even during the meal. The sequence is that way so that you are testing BG before food has an effect on it, then feeding (which will make BG rise), and then shooting. This allows you to make sure kitty has eaten well (hopefully) before giving insulin so that you can be safe doing so. If kitty doesn't eat, then you have to either skip or reduce the dose, or you have to delay the shot until food is consumed in order to avoid exaggerated effects from the insulin (hypo for instance). Skipping, reducing dose and delaying shots - none of those options is really attractive. So most people try to feed the major part of kitty's daily food intake just before shooting insulin. Basically, you want to make sure kitty has food in his/her belly when it's time for a shot.
Of course, kitties don't always cooperate, and they live on their own little schedules in their own little worlds! So sometimes us Beans have to adjust accordingly.
Carl