In my 35 years of radio I have never really found the internal SWR meters to be that far off really so apparently I am in the minority. One thing to consider when looking at an internal and external meter at the same time and seeing two different values of SWR is that if the antenna is not a pure 50 ohms then the SWR will appear to be different values at different points along the transmission line. Since the external meter is not located at the same point as the internal meter they will read different values. You will see the same thing if you simply insert an extra couple feet of coax cable.I suspect that the internal meter is fine and is simply measuring the SWR at whatever point it is in the circuit. If you insert another meter inline then the point where the internal meter is gets changed and thus the SWR reading will change.
My problem is that hooking up the external meter changes what the internal meter is showing. I have many radios and I consistanty see a slight variation between the external and an internal meter. This particular system is the first time I've seen the radio's internal meter change its reading by adding the external meter into the system. If the radio's meter shows 1.6 with the absence of the external meter; then when I add the external meter, the radio's should still read 1.6. But the radio meter changes to a 1.4. I see the radio's internal power meter change its result as well, going from showing a 20W DK with no external meter hooked up; to the radio's internal showing a 30W DK with the external meter hooked up. It doesn't make sense.