After a closer look at your example, I think you are counting the mismatch loss twice.
What you call transmission loss is:
- Transmission loss = S21 - load attenuation
And there, I believe transmission loss already includes the mismatch loss (it’s all the losses between port 1 and 2, minus the load attenuation), such as
- Transmission loss = mismatch loss + transformer loss
If you add missmatch loss to transmission loss, you count the mismatch twice.
Maybe ?
Edit: I found a post from you, where you explain it as I believe it is.