This image is the result of the exact same file, one rendered on my computer and the other on sheep-it. The quality of lighting in the sheep-it version is superior, but I cannot understand what it actually is.
The local render is GPU (CUDA) but I tried a small part of it on the CPU (because it would take weeks to try rendering it all) and the lighting is the same. I have also tried Windows on the same computer and a Mac.
For a specific difference look at this detail. There is distinct difference in the shadows, as is on the upper bar shelf.
Reducing the intensity of the overhead lights does introduce those shadows but changes the overall image in other ways. Plus changing the value of one node in one material doesn't make sense. It's probably some global variable that is different.
Blender version is 2.78a downloaded from blender.org or the Thomas Schiex ppa (same result).