Its good to give factual numerical values. But I looked up GWWCs explanation for the suspiciously round number of 10% and it is:

Why 10%?

We chose 10% because it strikes a good balance. It is a significant proportion of one's income, in recognition of the importance of the problem and the need to take real action. But it is also within reach of most people in the developed world.

There is also a strong historical connection to the idea of tithing, a tradition in Judaism and Christianity of giving 10% of your income to charity or the Church. Islam has a similar practice (zakat) in which those who are able give between 2.5 and 20% to the poor and needy.

The pledge is of course just a minimum. Some members decide to go further than this and pledge to give a higher percentage, such as 20% or even 50%.

So this 10% appears to be arbitrary from the point of view of the OPs question. There seems to be no ethical reasoning behind the 10%. At least it looks more like charity-optimization. This may sound harsh, but this is what it looks in this light.