After receiving a replacement for my defective 705 I questioned why it displayed three different altitudes in three different places, the Location, Elevation Change graph, and Diagnostices screens. Also, why it gave different readings each time the unit was turned off and back on even though the unit had not moved and there had been no weather change.
Here is Garmin's answer:
"Each time the 705 is turned on it is looking for a new elevation reading regardless of geographic location or how long the 705 has been turned off. This helps the device avoid any bias from the previous time in case the elevation reading was off. If you ride at specific location often like your house, using the waypoint calibration is the best way for you to avoid having to wait for the device to get a good lock.
The logic being that if you were riding and the ambient pressure happened to change (IE a storm front), you would no longer be accurate as to your current elevation. If you turn the device off and back on it would not use that incorrect information.
The 705 does do sampling from both the barometric altimeter and GPS at all times but only records from the barometric altimeter. The location is from the GPS and elevation graph is from the altimeter. As for the diagnostic page, that information is proprietary and I could only speculate where that is being taken from."