Recent evidence suggests that glucose depletion doesn't seem to affect the brain. For example, [1]. Moreover, the calorie consumption of the brain is basically constant regardless of what it is doing. I would hazard an untrained guess based off of these that the difference is in changes due to the body knowing of consumption of glucose versus any direct effect of the glucose itself. See also the study that athletes performed better after swishing a sweet drink in their mouth and spitting it out just as they did when actually drinking it.

[1] http://www.epjournal.net/wp-ontent/uploads/ep08244259.pdf