I don’t think the whole round trip latency is been measured. It wouldn’t make sense since Cubase knows the input and output latency and can compensate this accordingly. What is actually measured here is the delay that might be produced inside the curcuit of your outboard gear. Cubase can not see this additional latency as it is not reported to it.
Here is what i think happens if you ping your external fx.:
lets say your input and output latency of your interface is 6ms for each. lets also say your outboard gear doesn’t produce any latency by itself. Cubase sends a ping through the outboard device and according to the latency the drivers reports it will expect the ping back in 12 ms (6ms out, 6ms in). This latency Cubase can compensate because it knows it. But what if the outboard device will lets say have 3ms delay of its own. Cubase cant see this. It takes now 15ms to get back. Cubase knows that these addtional 3ms must come from the device. It should show you about 3ms delay compesation now.
or in other words. If your outboard doesn’t produce latency by itself, additional delay compensation will be 0.00 ms for your external fx. I would assume that pretty much any analog outboard gear will result in no further latency and therefore the result will be 0ms additional delay compensation.
I might be wrong here but this is the only thing that makes sense to me.