[mileswu]

This is how I lean too. A few percent in a mass measurement is a pretty small systematic error and could easily come from something that has been overlooked or miscalibrated. There are a tens and hundreds of calibration and corrections that go into these detectors and analyses and it's all too easy to miss something.

What really makes me convinced it is an experimental error is that CMS does not see the same mass discrepancy. If both independent experiments saw the same discrepancy then this would truly be really interesting new physics.

Disclaimer: I am on ATLAS myself, though not associated with the Higgs groups. I can only comment on public results due to much information being confidential.