Kernel upgrade 7.20 NW 7.02

Hi,

We are looking to upgrade our kernel to last version as 7.20 will not be supported after Q1 2015.

When checking PAM, we can only see 7.21 as supported.

I found this link that explain the minimum kernel version of 7.4 depending on SP Stack level.

Understanding Kernel Releases for the SAP NetWeaver AS ABAP

What I cant understand or find is why we cant use a upper Kernel version with a 7.02 NW?

Do you have some links or explanations to provide?

Regards,

Adel