Barometric shenanigans, perhaps?
Hum. I've always considered the 30x barometer a needless complication. Maybe I'd better have a gander.
Bit late replying to this, oh well. In a E30 or 30x, you basically have 3 options for viewing/recording elevations.
* The factory default and most commonly used mode (
and best, IMHO) is Barometer mode 'Variable elevation' and Auto calibration set to 'On'.
* An alternative would be the same mode with Auto calibration set to 'off'. Then of course you have to periodically calibrate it yourself which might be workable in very stable weather conditions but generally in British weather would be far more trouble than its worth.
The Auto calibration may not be perfect, and can be bizarre for a while after switch-on, but I'm willing to bet it will give better results overall than any manual calibration regime.
In either of these modes the elevation you see, in any data field displayed on any page**, and the elevation that is recorded into the Tracklog, is derived from the barometer.
**There is one exception, which is the elevation displayed on the 'Satellites' page - this is always the GPS-derived figure.
* The 3rd option is to turn the barometer off - well actually you can't, but you can set it to 'Variable pressure'. In this mode the elevations you see, on any page, and the elevation that is recorded in the Tracklog, is GPS-derived.