AD-FMCOMMS4-EBZ layer 5 and layer 6 ground differences

Hi, I am designing custom AD-FMCOMMS4-EBZ board. There are many differantial pair in layer 5 and layer 6.  Layer 5 dont have ground  in differantial pairs namely it is not coplanar.  you can see attached pic. But  differantial pairs in layer 6 has ground that is it is coplanar. what is the reason of doing this type of grounding.

[edited by: gcetinkaya at 9:39 PM (GMT 0) on 14 May 2020]