Time Sensitive Networking (AVB) delays measurement

Hi,

I am testing the Intel I210 TSN Card (AVB). By measuring the Propagation Delays between a PC and a switch, it appears that the values are lower for 100 Mb/s speed link than for 1 Gb/s speed.

Link Delay             PC-> Switch    Switch -> PC

1 m cable: 100 Mb/s          86 ns          82 ns
              1 Gb/s         220 ns         212 ns

25 m cable: 100 Mb/s         194 ns         192 ns
              1 Gb/s         305 ns         303 ns

It's quite strange. I wonder if this result comes from the RX and TX PHY compensations applied to the Timestamps.
Indeed, PHY compensations defined by Intel are :

                     100 Mb/s      1 Gb/s
TX PHY Compensation:   1044 ns      184 ns
RX PHY Compensation:   2133 ns      382 ns

Questions are:
- How RX & TX PHY are computed?
- How we can explain the above measured values?
- What is the "correct" PDelay?

Thanks to answer if you have done the same type of testing ou have ideas about that.

Have a nice day.

Patrick