Good question!
I say that it has a lot to do with how the shape of the key way controls the type and amount of tension that is applied to the plug. Depending one whether you are turning the lock clockwise, or counterclockwise, the type of tension wrench, the and the dimensions of the key way all regulate how the pins will be receiving binding tension from the lock plug + lock body.
If you are having trouble picking a lock in a certain direction, mess around with the tension a bit and continue to try.
One thing that you mentioned, is that you had to "unlock it with the key..."
I have to admit, that is lame, I am sorry to hear that!

Keep practicing.
Also, there could be other things going on inside this lock, could you please post a picture or describe the make/model so we could get a clearer picture of the style of lock you are referring too.