I hope you understand haha. Look at your own hand. Try to model this "triangle" and make the thumb go slight downwards. On your model, the thumb is going straight out of the side of the hand. That will result in that the hand will look really weird if the hand is gripping something or bending the thumb...
You should put the thumb more in the palm of the hand... Or how should I explain.. If you are going to bend or twist the thumb as you have modelled it now, the whole hand will be deformed.