Not to be mean, but I find it kind of sad if your instructors couldn't explain that to you :)

You probably know that when an aircraft stalls it is because the angle of attack becomes so high that the airflow starts to separate from the top of the wing and the wing is stalling.
When pulling negative G's, you will have negative angle of attack and this separation is just from the bottom of the wing. It is probably easiest for you to imagine this if thinking of an aircraft flying level inverted and therefore subjected to -1 G. (Every thing is just reversed)

With an asymmetrical profile the separation will happen earlier with negative G's compared to positive G's