msl_hght in PVT data

I have been experimenting with downloading PVT data from Garmin GPS
devices.

The D800_Pvt_Data_Type contains two height fields

float32 alt; /* altitude above WGS 84 ellipsoid (meters) */
float32 msl_hght; /* height of WGS84 ellipsoid above MSL(meters)*/

In the Garmin Device Interface Specification (IntfSpec.pdf) it says:

The “alt” parameter provides the altitude above the WGS 84 ellipsoid.
To find the altitude above mean sea level, add “msl_hght” to “alt”
(“msl_hght” gives the height of the WGS 84 ellipsoid above mean sea
level at the current position).

I tested this with 3 Garmin devices at the same location (in northeast
Hampshire, UK).

The GPS Pilot III and GPS Map 196 both gave msl_hght values of
-157.88ft (note: negative value). The resulting value of alt +
msl_hght was in good agreement with DTED data for the same location.

However, 2 GPS 18 devices gave msl_hght values of +155.20ft (note:
positive value) for the same location, and the resulting sum of alt
and msl_hght was thus about 300ft too high.

The Garmin GPS 18 Technical Specifications
(425_TechnicalSpecification.pdf) describes the msl_hght field as "Mean
Sea Level Height (meters)".

It appears to me that the GPS 18 is reporting msl_hght as MSL height
above WGS84, whereas the other 2 devices are reporting height of WGS84
above MSL.

Can anyone confirm this?

Does this just affect the GPS 18, or are other Garmin devices also
affected?

If so, is there a way to query the device to determine the sense of
its msl_hght values, or do you have to hard code it?

I am using PyGarmin on Red Hat Linux.
thank you,
--
RT