It is the way Wisconsin calculates tax on part-year income.  Wisconsin is not taxing all of that income.  However, Wisconsin pretends that all of your income is taxable according to Wisconsin law (and your full deductions are also claimed according to Wisconsin law), the tax is calculated on that amount, and then the calculation is prorated to the amount of income actually earned in Wisconsin.  You should see a percentage calculated on the Wisconsin nonresident return that represents the difference between all of your income and what was earned specifically in Wisconsin.

However, not all of your income will actually be taxed in Wisconsin.