I'm starting a new project where I want to re-use the physical 16x PCIe connector. Since I will also have a lot of differential signals, I looked over the PCIe usage of the pins, and now I'm wondering about the number of ground pins the standard uses. In the higher numbered pins, it is very consistent with ground always opposing signals. For example, A66 and A67 are transmit lane 12 so the opposing B66 and B67 are ground. Then A68 and A69 are ground and B68 and B69 are the receiving lane 12. Transmit lane 13 then takes A70 and A71 with ground on B70 and B71 and so on. On the lower end, the pattern isn't always followed. Transmit Pair 0 takes A14 and A15, but B14 is part of the reference clock pair.
Why are the signal pins opposed by ground? I get having a ground pin between the differential pairs on the same side, but wouldn't the inner ground layer of the PCB provide enough isolation between the A and B sides?