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Open with Blender Hash:698a9a2
Volume changed from Distance to Equiangular.
Looks like new multiple importance was added before the two others, shifting their num id by one? Should be fixed or handled in versioning code I guess?
This is not a bug. The Volume Sampling method was a general render setting in 2.71, now it is a setting per world and per material. In other words, it's a intended change.