The income I get taxed on is not the same as the one reported on my income document

The main reason is that you might have part of your income which is not considered as taxable under the IRC (Internal Revenue Code) or the tax treaty benefits that you were eligible for (please refer to Step 3 - Let's talk money - Tax Treaty verification).

Also there are other applicable deductions and exemptions that can reduce your taxable income such as the Itemized deductions (your state and local taxes paid).