Why is the SWR reading for my 4391A is high and not matching up with what I calculated?

While you may have reviewed the troubleshooting tips in the manual, one area that tends to get overlooked is the following detail in the "Reading SWR" section in the manual:

So if you are pairing a 100H for forward with a 25H for reflected, then there is a good likelihood the SWR displayed on the 4391A will be off.