I'm just guessing, but if you honestly reported all your earnings I'm guessing you didn't earn enough to have to pay taxes. That's one reason you wouldn't have to pay taxes even if you didn't have any withheld.
Another factor might be that you are eligible for an "earned income credit", which people are sometimes eligible for if they have children, have an income under a certain amount, but have worked and earned income in the past tax year. There's a chance you could have been eligible for earned income credits for each child.
The fact that you included your expenses may account for some, if not all, of the expenses you reported making your income essentially less than it really was. Usually, if you use something like an online tax preparation "outfit" the questions asked about your expenses are the ones that count. The same applies to questions on the tax form you fill out.
You can call the IRS and get a copy of your return if you want to see how things were broken down. Also, if you used an online service you should be able to get a copy from their site, by copying and saving the return from there.