Why two LPU's needed on PTP600

How come on the PTP600 it requires a LPU on the bottom [u:2m32b9da]and[/u:2m32b9da] the top but the PTP500 and PTP400 only require a LPU on the bottom?

The PTP 300/500 Series has Lightning Protection built into the ODU, the PTP 400/600 does not.