Usually due to discrepancies in Google’s terrain elevation data. I’ve seen discrepancies of 40-50 feet vs actual survey data supplied by USGS. Combine that with pressure changes due to atmospheric conditions and wind on slopes or terrain features, and its effect on the barometric altimeter, and you can easily fly an aircraft into the ground.
When planning survey flights where the aircraft must fly at a specific height above terrain I ALWAYS use the ArcGIS elevation model from the survey done by USGS, calculate the waypoint altitude myself, and never use Google’s estimation of terrain elevation. Google’s stuff might be “good enough” for the average consumer. But it’s not good enough to fly an aircraft with. If you read Google’s licensing terms on their supplied data, that is clearly stated.