Most of the important points have been well covered, but let me add a
general observation:

Loss will skew any measurement toward the characteristic impedance of
the cable. This is intuitively evident, since if the cable is lossy
enough, you'll see its characteristic impedance at the input regardless
of what's connected to the other end.

So when you say you're seeing 74 ohms when you should be seeing 105, I'd
bet without further evidence that the cable has more loss than your
model thinks it has.

Roy Lewallen, W7EL