Stryd uses a barometer to determine elevation. We have not GPS onboard or other means to measure elevation. That means that Stryd is really accurate for immediate elevation changes (i.e. second by second). However, if the environmental conditions change, such as air pressure, the absolute elevation can differ from actual, and as air pressure can change even during a workout, elevation reading can drift within a workout.
GPS is more accurate for absolute elevation and is not affected by air pressure, a barometer is more accurate for immediate elevation change.