According to Garmin, the following can affect the elevation data on a 1030:

“What are some of the factors that can cause an elevation reading to be wrong?

• Where the device is mounted on the bike
  • Garmin recommends the use of a Garmin mounting accessory placed near the handlebars of the bike
• Weather conditions
• Group vs. solo riding
• Road traffic
• Covering the device with stickers or a case. If you cover the barometric port on the device which is tiny hole generally found on the back or sides of the device this will negatively affect elevation accuracy.

The barometric altimeter is not temperature compensated. Temperature changes in the measuring device will affect the barometric pressure sensor and altimeter readings.“

I’ve no clue why the unit mounting location, nor the riding solo vs, group ride, would affect the data, but that’s the official Garmin explanation. Obviously weather and temperature will affect the barometric sensor where as the actual GPS elevation data will be all over the place, cause GPS receivers in general, are notoriously inaccurate at determine elevation.

Thus I’d say the unit is totally in error.

If you want a better sense of what the on-line route creation data is stating, run the route thru different programs, RWGPS, Connect and Strava, as example. I’ve seen differences between RWGPS and Connect, but only 60-70 ft. In 3500 or so and I chalk that up up how the particular program reads the map data.