To respond to the original posters question. I have often asked the
same thing. My observations are this. My plane is sitting in the hangar
at 6412.37 msl. The airport was just recently surveyed so I am sure
this is a real close number. I set my altimeter to the current setting
and it shows within 3 feet. I fire up my King KMD150 MFD and it will
almost always show 8- 12 feet higher then that, never seen it lower. It
must be that mathmatical modeling thing. Then I fire up my Garmin 196
and it varies between 15 feet higher and 10 feet lower. I attribute
that to a less sensitive circuitry in the handheld unit altho I might
be wrong surmising that. I would be curious to see what others see in
their respective areas.?

Ben
www.haaspowerair.com