I honestly don't know the exact reason why but I'll guess.
Don't know about the "classic" but certainly on the NG the no3 window is heated, however as opposed to the no1 & 2 windows the heating on 3,4 & 5 is via a conductive coating on the inner glasspane.
This is not to increase the elasticity of the window in the event of a birdstrike but more as a anti-fogging device. A birdstrike on the no3 or 5 window would probably be more accurately described as the bird having a planestrike. The no 4 window on the other hand is more forward-facing and because of that it may have been given the extra shatter-proofing vinyl layer.
I stress to add that I'm only speculating now.
Regs