This is an issue (or feature?) I experience with the blender version 2.76b from the offical ubuntu 16.04 xenial xerus repository. I can't reproduce it with 2.78c or 2.76b build from the blender homepage.
This one is rendered with the xenial release. Almost no noise and fast render time.
This one is rendered with CPU and yields the same result as the same version build from the blender homepage. Both four times higher render time and much more noise.
Is all this maybe due to some specific build options? Because i'd really like to keep the first result even in future blender releases.
It gets even stranger when I decrease the samples from 60 to 10. Xenial build GPU shows multiple sharp shadows around the object while the Xenial CPU and blender build CPU + GPU gets just noisier.
The file from the examples is unchanged between renders, you can try it yourself but the condition occured with every blend file I stumbled upon, therefore it shouldn't be file specific.