[Elecraft] P3 TxMon - SWR readings adrift

[ANDY NEHAN]

Hi guys, I have just received the P3 TX Mon (easy to install and get working). I also have an LP-100A which is in series right next to the monitoring head of the P3 monitor. The KPA500 and the KAT500 both indicate an SWR of between  1 and 1.2 and the LP100A indicates an SWR of 1.05 into my 500watt 1.3GHz 50 ohm dummy load. However the P3 TX mon says SWR of 1.67. Now it is not a cabling issue as it does not matter which way round the monitor heads are (the LP100A head either before or after the P3 Mon head) the results are the same.
My question is why the big difference - I had every expectation that the P3Mon would say something very close to that shown by the LP100A, the KAT500 and the KPA500.
Any ideas as to what is going on. BTW my P3 firmware is uptodate.
Andy G4HUE
K3 serial 6990
P3 serial 2493
KAT serial 1245
KPA500 serial 2102