Paul,
1. Because unless you have a certified Baro-VNAV system, you aren't flying an LNAV/VNAV approach. As has been indicated already, you are flying an LNAV +V approach.
I'll try and explain simply why you are getting advisory and not guidance. So procedures are coded in ARINC424, which includes altitudes at certain fixes. The Garmin boxes do know what altitudes to display as they fly the approach. The problem is that, without a certified BaroVNAV system, the Garmin only knows its height reference to the WGS 84 ellipsoid and not AMSL. The difference between these two datums varies across the country but generally the WGS84 ellipsoid is ~70m below AMSL in the SW of Aus and ~70m above in the NE . If you want to nerd out you can read more about this here, which explains the rough idea as it applies to GDA20
https://www.icsm.gov.au/upgrades-aus...ference-system
So in a nutshell, the Garmin navigator (on its own) doesn't use AMSL as its reference datum for altimetry. The difference between WGS84 ellipsod and AMSL can be calculated, and I am aware that Garmin use a model to do these calculations on the fly. This eliminates this error, however this functionality is not part of the altimetry certification of the navigator and so can only be used as advisory. This is why most people are not observing ~150ft errors when using these systems.
The introduction of an SBAS system will also remove the altimetry errors, and as SBAS a certified system you will be able to fly an LNAV/VNAV as guidance (and coupled). Further the WGS84 ellipsod - AMSL difference can be hard coded for the threshold you are flying the approach to and a geometric 3d vertical path can be generated. These types of approaches, will be known as LPV.
Alpha