There is an element of physics which everyone seems to overlook.
Let us take a look at the QB's we have played this year:
Drew Brees 6'0
Bruce Gradowski 6'1
Fitzpatrick: 6'2
Colt McCoy 6-2
Mark Sanchez 6'2
Chad Henney 6'3
Jimmy Clausen 6'3
Tom Brady 6'4
Matt Ryan: 6'5
Vince Young: 6'5
Jason Cambell 6'5
Carson Palmer 6'5
Kerry Collins: 6'5
Josh Freeman 6'6
Joe Flacco 6'6
Now, I am not going to list all the LB's, but it should be known that most of the linebackers are between 6'2 and 6'5.
That means, a good form tackle would put the average LB chest to chest with a wrap and tackle.
Now, let's remind ourselves that James Harrison, IS 6'0!!
Which means, when he goes to tackle, when he leans in to hit, the top of his helmet will NATURALLY be at the jaw/lower level face of literally HALF the QB's. Thus, the NATURAL and NORMAL way to tackle, will ALWAYS cause a helmet to helmet hit on half the QB's played this year.