They have now introduced the 'Commerce Expansion Adjustment' whereby some sales are made through methods such as carrier billing and the developer will only get 56.1%.
There is no option for developers to opt out of this type of sale; Microsoft suggests that 'You may want to consider if the Commerce Expansion Adjustment applies in a country/region where your app is available and factor that into your market pricing strategy'
As a veteran of the store pricing wars on Palm OS, I have seen how this plays out. Stores competed to sell through new partners, and offered increasingly large shares of revenue to those partners. Inevitably, that came out of the developer share.
I would be very happy for Microsoft to offer me an option to make additional sales at a lower revenue share — but I'm not happy at being forced to suck it up.