On all the rides that I have done over the years, it is very rare to get every rider to have the same elevation, even comparing to Wahoo's etc. But, I usually find my Garmin is almost right.
What I do is to setup a waypoint at the corner of my house where I have to pass over when I start my ride or if I can, close to the start point of an event. I also have recorded with the waypoint, the elevation of that spot I pass over and stop on as I press the "Start" button. A small window will then pop up saying "Calibrating elevation" etc and something else.
Another thing that can affect it is changes in Barometric pressure if there is a sudden change and sometimes I wonder if air pressure over the device from disturbed air from the bars can affect it especially in gusty windy conditions or fast downhills or both combined. I know a few months back, I found that I could alter the elevation reading on my 530 by blowing over the bottom of the device.
