If my salary on my last check is bigger than my income before deductions on my w2?

[impj2]

My salary is  say for example 50.000 on my last check and my gross income on my w2 (before deductions) is lower.

[Opus 17 · Accepted solution]

Most likely, some of your payroll deductions are pre-tax and reduce your taxable income, such as health insurance premiums, retirement contributions, FSA, HSA, and a few others.  You would have to ask your payroll department for the exact answer that applies to you.