This is just my theory, but as good as any seeing as no-one outside the Home Office really knows..
I think there is a list of nationalities which automatically incur the entire gamut of external checks, which, political niceties aside is easy to understand.
In addition to this list I reckon they have some kind of "1 in 10 full strip search" rule, and those unlucky applications are going to take longer to process.
As to the checks themselves, hell who knows. As much as possible from computer systems obviously, and some applicants from 'friendly' countries (eg Commonwealth) where the data back home is quickly accessible are going to get processed faster than say, a country like Zimbabwe which doesnt even have properly computerised Immigration control.
But thats all just a guess
If you always do what you've always done, you'll always get what you've always gotten