At the moment Google has one search index for both mobile and desktop, you get similar results when you search on the different devices, that it apparently about to change.
According or a recent report from Search Engine Land, Google is building a new search index which will be for mobile only and this will apparently be more up to date than the desktop version.

The new mobile index will be Google’s primary index and the desktop index will be their secondary index.
Exactly how this new index will work has not been revealed, it is not clear as yet on whether it will only show mobile friendly results or whether it will show both. Google is expected to implement these changes over the next few months.
Source Search Engine Land, TNW