Why are earthquake magnitudes almost always revised downwards?

It seems like every earthquake I've seen has started out at a slightly higher magnitude than it ends up at after a few hours or days of revisions to the initial estimate.

I can't say for certain if that trend holds throughout the world — I haven't looked at a comprehensive dataset or anything, and I live in an area where earthquakes are strike-slip rather than normal or thrust — but it seems odd. If a typical initial estimate for an earthquake is almost always 0.1 or 0.2 higher in magnitude than the final analysis shows, why is there not calibration to align the initial estimates more closely to the actual values?