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Now that your tax paperwork is showing up in the mail (or email), are you confused by the earnings reported on your W-2 wage and tax statement, the summary of your income and taxes paid that your employer sends you every year.

Not to worry. "It's actually a good thing to have your W-2 wages be less than your salary as this means you'll owe less taxes," says CPA Amy Wang, senior technical manager for the American Institute of Certified Public Accountants.

What you see on your W-2 is your taxable income, not your total salary. What's more, the amount shown in Box 1 for "wages, tips and other compensation" may be different than the amount in Box 3 for "Social Security wages" or the amount in Box 5 for "Medicare wages," and all three sums may be less than what you actually earned in 2015.

Here's what's going on.

Lower-than-expected taxable wages mean you were successful at sheltering your income from taxes over the past year. This difference between your actual and taxable earnings typically arises from four different situations:

You contributed to a company-sponsored retirement plan: Saving in a retirement plan like a 401(k) reduces the amount of federal and state wages you will be taxed on, which are reported in Boxes 1 and 16, respectively.

You get health insurance on the job: If you participate in your company's health insurance plan, you pay your portion of the premium with pre-tax dollars. Your taxable wages (Boxes 1, 3 and 5) are reduced by the total of the premiums you paid.

You fund a FSA or transportation reimbursement account: The pre-tax money you have deducted for a transportation account to pay for public transit or parking and a health care or dependent care FSA will lower your taxable wages that appear in Boxes 1, 3, and 5.

Your company pays you back for mileage: You may have non-taxable reimbursements show up in your paycheck that are not strictly salary, such as being reimbursed for mileage you incur. Those are not part of your taxable wages.

Read Next: When Will I Get My W-2?

The other mystery on your W-2 is why your Social Security wages differ from your actual pay. If you earned less than $118,500 in 2015, expect your Box 1 wages to be less than Box 3 and 5 wages, says Wang, since you can shelter more of your earnings from federal taxes than you can Social Security and Medicare taxes. If you earned more than $118,500 last year, you ran up against the Social Security tax cap, which is the max income you pay Social Security taxes on. In that case, your Box 3 wages will likely be less than your Box 1 wages.