I stumbled on this very inconsistency while working on some volumetrics where a notable difference in render results for CPU vs. GPU became visible. This effect appears for both Path Tracing and Branched Path Tracing for all types of lamps, and it gets more prominent on low sample rates. This difference is too strong to be acceptable when using a render farm with a mix of systems.
Following both images have been rendered with exactly the same settings on CPU top and GPU bellow for 1 SPP (so should look the same):
Here with 2 SPP:
Here is the blend file to test:
Tested with latest official build bot release on Win7 64.
Hardware: i7-980X, GTX 480