Why do some people owe money at tax time, while others get a refund?

How is it that with a single income of less than 30,000 in Canada that I owe the government money? How come friends and family who make more or and some who make the same get a huge refund?
Update: What are the calculations based on? Is there a link that shows why you are being deducted and what for? The CRA website is not very helpful at all, I searched and cannot find anything explaining why they take what they take from you, on top of the money they take off each paycheck. This country is a joke. Steals... show more What are the calculations based on? Is there a link that shows why you are being deducted and what for? The CRA website is not very helpful at all, I searched and cannot find anything explaining why they take what they take from you, on top of the money they take off each paycheck. This country is a joke. Steals all our hard earned money.
Update 2: How come I underpaid, isn't that the job our our payroll person. To make sure enough taxes are taken off? It's not fair, and who controls that. Where do you see the amounts you must pay? Is there a list that the CRA goes by? How to you determine what one person pays versus the next? Show me?
Update 3: Thank you John K. Finally a place to check. Stupid employer is not taking the right deductions as I have only 1 employer.
Update 4: Oops I mean John Q
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