What's wrong with Tom Robinson's arm in To Kill A Mockingbird?

During the
trial, the sheriff, Heck Tate, testified that Mayella was beaten basically on her right side. 
This indicated, even to , a girl of nine, that someone who was left-handed would have done that
kind of damage.  Whenproves that Bob Ewell is left-handed, Scout says,

"Atticus was trying to show, it seemed to me, that Mr. Ewell
could have beaten up Mayella.....If her right eye was blacked and she was beaten mostly on the
right side of the face, it would tend to show that a left-handed person did it."  (pg
178)

However, at that point in time, she thought that
maybe Tom Robinson was left-handed.  That is until Atticus asked Tom to stand up.  Scout noticed
that something was definitely different about Tom Robinson physically, he seemed off
balance. 

"His left arm was fully twelve inches
shorter than his right, and hung dead at his side.  It ended in a small shriveled hand and from
as far away as the balcony I could see that it was no use to him." (pg 186)

Tom Robinson has no use of his left hand. Atticus was tryingto show
that Tom could not have done the damage to Mayella's face. also noted the crippled condition of
Tom's arm.  Reverend Sykes, who had snuck the children into the balcony to watch the
proceedings, explained that

"He got it caught in a
cotton gin, caught it in Mr. Dolphin Raymond's cotton gin when he was a boy....like to bled to
death....tore all the muscles loose from his bones." (pg 186)

The point is emphasized again when Tom is called to testify and tries to take the oath
on the Bible.  He has to lift his left hand with his right one, and then it won't stay in
place.  Finally the judge tells him to just take the oath and not worry about placing his hand
on the Bible.