I can't say with absolute certainty that's what is really going on, of course; but on my setup, which I'm confident is pretty accurate, it sounds subjectively much too loud in the rear channels, that's for sure. 3 db of gain amounts to about a 40% increase in volume, which is a pretty big difference. It's not subtle.

The difference in movie theatre calibration arises from the days of mono surround when the entire surround array was calibrated at 85 db to match the front channels, and the introduction of stereo surround meant splitting it in half each at -3 db, to maintain constant acoustic power when the halves were summed together. This was done to maintain backwards compatibility without having to adjust the levels differently depending on the audio format of an individual film. Since that calibration standard has remained unchanged (other than for Imax which uses single speakers for each rear channel the way home theatre does), 5.1 mixes are pretty much always going to have their rear channels 3 db higher if they were mixed on a typical recording stage, and thus require the appropriate reduction in level when being transferred to home video. Though it usually is done properly, mistakes are sometimes made.

Apparently some early laserdiscs with 5.1 sound simply ported over the theatrical mix directly without recalibrating, and would thus have this same kind of problem. It is also possible that some don't get lowered because someone in charge thinks it actually sounds better that way. The SE mixes of ESB and RotJ seem very overbearing in the surrounds too, and in that case I suspect the tin ear of a certain Matthew Wood is to blame . . .