This is probably another question with a very obvious answer but it's puzzling me.

I am just building a character specialising in Greatsword as a weapon.

With the Improved Critical feat and seven levels of Weapon Master I get a critical spread of 15 - 20. One better than I expected and very welcome too.

With a keen Greatsword the critical spread rises by a further 2 points to a massive 13 - 20.

Is there some bonus or multiplier at work here that I've missed?

The greatsword has a range of 19-20, i.e. two points. Keen, Improved Critical, and Weapon Master can all of them double that range, i.e. add two more points to the threat range.
They don't multiply one another's effects though, so you get 19-20 for none, 17-20 for one, 15-20 for two, and 13-20 for all of the above.

For the kukri, rapier and scimitar the range is 18-20 without modifiers, and 9-20 with all three, making them the perfect weapons to inflict critical damage with.

Yeah...

Hides face in embarassment.

Must read the manual with more care in future.

I have not used Greatsword before and the two-point gap between a keen and non-keen weapon was confusing me.

Thanks for the explanation.