I found on a recent flight that my Skyview was showing altude 300 ft higher than my backup, and ATC said my Xponder was showing the same. Today I went to the hangar to investigate this and found that when set to the current altimeter, instead of showing the normal 980' msl the Skyview was showing 1500'. I found the calibration in setup to correct this, but have since read Dynon's comments about systems not leaving the factory unless they are within 15'. What could be causing this change?