The first couple of pages show how much better a full two port calibration is. See figure 1 that shows a full two port cal vs the response cal. The response cal has a lot of ripple on it. However, the ripple can sometimes be a lot worse than this.
But the Siglent SSA3000X-R will be doing the full two port calibration named "Short-Open-Load-Through (SOLT)" above figure 1?
Maybe we have a misunderstanding. It can only transmit on one port but it will receive on two ports. It will therefore do a two port through calibration just fine. Naturally in addition to the short, open and load calibration.
OK, I can try and explain my point. I think the Siglent SSA3000X-R is a T/R VNA and it can do something called an enhanced through calibration. This corrects for the port 1 mismatch but this isn't as good as a full 2 port calibration because it can't correct for the port 2 mismatch. Port 2 won't be a perfect 50R port. It will have some mismatch.
By contrast a genuine full 2 port VNA can typically do a 12 term correction that includes correcting for port 2 mismatch.
If it helps, the red trace in the plot below shows what a perfect VNA could measure (in terms of S21) for a fairly short 25 ohm transmission line between port 1 and port 2.
The red trace is close to what a proper lab VNA would show if it supported a full 2 port calibration. This is the correct response.
If the T/R VNA had an uncorrected port 2 VSWR of 1.3:1 (but everything else was perfectly calibrated correctly) then the best you can hope for is an uncertainty window represented by the blue trace.
Can you see how grim the T/R VNA performance is in this comparison? It is possible to add an attenuator at port 2 of the T/R VNA to improve things but this comes with the penalty of reduced dynamic range.