I think the situation is that there are particular and quite rare circumstances in which this happens.
I suspect that the Government has been unwilling to comment on what the figures are for 3 months tells you what you need to know - especially when the 6 month figure is under 6%
With regards to people from outside of the EEA this is covered by
Section 115(1),(3) of Labour's 1999 Immigration and Asylum act
Those from inside the EEA have to pass a 'habitually residency' test
http://www.housing-rights.info/habitual-residence-test.php
The DWP link to that is currently broken (interesting)
I rather suspect it mean you have to have been in the UK 3 months in most cases