Being a curious type, I decided to see if I could find out why! And my comment turned into a post (thanks Aliqot!):
In their discussion of the politics of women in Northern Ireland, Anne Marie Gray and Deirdre Heenan suggest these countries have strived for increased female representation on three main principles:
First is democratic justice- that justice is an important principle, and that it is unjust that women are under-represented on decision-making bodies.
Second is resource utilisation - that valuable human resources are wasted when half the population is not involved in politics.
The third is interest representation - that because of the different experiences of women and men (in relation to economic and social structures) they have different political interests, implying that in politics women will employ a different set of values and pursue different interests from men.
Gray and Heenan also suggest that while women in the UK have the advantage of increased education, theoretically opening both political and professional doors, we remain at a disadvantage:
... entering the labour market has not resulted in a lessening of their domestic responsibilities. To many, the prospect of active involvement in politics must seem little more than a potential additional burden.
... "those elected will be peculiarly skewed to a certain kind of woman who, like the generations of men who went before her, will be a well-educated professional, and devoted to politics full-time".  Even in the Nordic countries, greater proportionality has not resulted in equal access for all women. We need to think beyond the numerical and to grasp the wider issue of representation. We need to think about how to encourage a more diverse range of women to put themselves forward, which involves rethinking women's role within the family.
The Fawcett Society concurs on the positive example of Scandinavian (or Nordic) countries but points out that Rwanda have an even higher representation of women in the government:
Women held 48.8% of the seats after last year’s election compared with a world average of 16%
How did this happen? The better question is: How was this achieved? The women of Rwanda collectively used their powers of intelligence, experience and determination to create the world they wanted to live in:
Rwandan women lobbied heavily, helped to draft a new constitution and developed voting guidelines that guaranteed seats for women candidates.
There are in fact a growing number of governments in sub-Saharan Africa that are following in Rwandan footseps:
In South Africa and Mozambique, for example, women hold about 30% of the seats in parliament - matching the international target set at the Fourth World Conference on Women, held in Beijing, China, in 1995.
The UK government flounders at 19.5% although the women in Wales and Scotland have had more positive results:
In both Scotland and Wales, women took advantage of the emergent devolved institutions to achieve new heights of political representation, winning 40% of the seats in the Welsh Assembly and almost 39% of those in the Scottish Parliament in the 1999 elections (Brown, 1998; Edwards and McAllister, 2001).
Ian McAllister and Donley T. Studlar, in Women’s Representation in Anglo-American Systems (Journal of Representative Democracy), also consider the political systems of New Zealand, the United States, Canada and Australia.