There's a thought is it the GPS that the phones and watches use to calculate altitude or do they have have altimeter's built in. Time for some googling.
I'm fairly sure that both Mapbox (now used by Strava), OpenStreetMap and Google all use the same digital elevation model for their mapping of Australia. It'll be the 10 metre DEM supplied by Geoscience Australia... primarily because it's free.
Now, if I remember the model parameters sufficiently, it's something like a 100 square metre pixel across which the average elevation is modelled based on terrain mapping and point source elevation data. I can't recall what the vertical precision of the thing is...
Each mapping software simply takes the GPS positioning and barymetric data off your device and then cross references it against the DEM in order to estimate altitude at a given point on your activity.
Given that none of these data points is especially precise to begin with then it should come as no surprise that everyone's final readings are quite a bit different. Or just plain wrong.
I used to ride around Mildura along the Murray River in NW Victoria...the area is roughly 40 metres above sea level and generally flat. Yet somehow it's quite possible to ride for a hour or two and claim 300 metres in elevation. And yet here I sit at 600 metres elevation on the central western slopes of NSW with the town at 550 metres and if I go for an hour or two ride I can claim...about 300 metres.
As a result...I don't take my Garmin or how Strava interprets the data it produces particularly seriously.