As far as I'm concerned the glycerol should form more hydrogen bonds with the water due to the greater presence of hydroxy (O-H) groups...
The only possible explanation which I can think of is that in forming the hydrogen bonds with water, multiple hydrogen bonds which already existed between glycerol molecules were broken (endothermic) and since glycerol molecules would have more hydrogen bonds to one another than ethanol would, it would result in a greater amount of heat being absorbed when initially breaking these bonds and hence less net heat being produced from glycerol when added to water than that produced by ethanol when the same is done.
Ofcourse this probably isn't the correct answer, but it may be worth writing if you haven't got anything else
Ask a teacher though.
/ 
