Which is the problem of my altimeter?

Description: 

Roundtrip warmup from point A to point A. Start run activity, the watch says "230m". Calibration with DEM says 215m (this is correct). Run the warmup and stop activity, I have this kind of elevation profile. Note that for the first half I run downhill and it register uphill...

After warmup I rest 2min and start a new run activity for the training at the same point A. Start activity it says "225m"...wrong, after DEM calibration it says "215m": correct! The training is a run from point A at 215m to point B at 209m, then back to A on the same path, then to B and so on for 8 times. I'm expecting to see a graph between 215m and 209m. Instead I have this!

Also this time the first segment from A to B (number 1) is 6m downhill, not uphill.

Suggestions?

Going for trekking I don't have these kind of problems