Actually you take any figures and the tax rate (B) after combining the spouses income is much higher than the individual tax rate (A) on their respective incomes.

For ex. both are earning 100,000 per yr, and their individual tax rate (A) is 17.55% according to bareme 2008, which is 17,550. Total income tax = 17,550 X 2 = 35100.
Now, if you look other way round, their collective income is 200,000 so the tax rate for married (B) is 21.85%, comes out to be 43,700.
I, personally, don't think this is the way taxes are calculated for married working couples taxed at source. Also, they can't fill the full tax return with all deductions etc, so what's the criteria employed by tax authorities to do the tax correction.