The way the tax system works is that different sources of income are combined such that your total income is taxed according to the rate schedule. So the Governor General has income that already takes him up to the top marginal rate, such that his existing sources of income are taxed through the rate schedule. Then it's the fact that his salary as Governor General would now become taxable, be included in his taxable income. Then by virtue of the fact that it would be added on to the top of his existing sources of other income, it would all be taxed at top marginal rates. It doesn't get taxed under the entirety of the rate schedule as a different source of income. It becomes all part of his own personal taxable income.
On May 16th, 2012. See this statement in context.