By using for analysis that 0.4, we are capturing the possibility of taxpayers using different strategies to try to avoid or delay their tax liabilities.
There are very simple and common ways to do it. For instance, if you're a small business owner, you can retain a bit more of your income in your corporation and decide to delay the distribution a bit. You can decide to postpone particular transactions that would have led to capital gains realization. You can decide to change your savings portfolio composition to a bit less interest income and a bit more capital gains and a bit more dividends. There are a number of ways you can do it without even appearing very aggressive.
You also have probably other moves. You can start using a bit more of some special features of the tax system that are allowed to you, either because there's a special credit or it allows you to delay the realization of income.