To expand on John's answer, Remember that the tax brackets are graduated, meaning that the first ~$9K of taxable income (after deductions and exemptions) for a single person is taxed at 15%, the next ~$28K is taxed at 19%, the next ~54K is taxed at 25%, and the next ~$100K is taxed at 28%, etc. So when you say you are in the 28% "bracket", that means that your annual taxable income will be somewhere between $91K and $191K. Every additional dollar you earn will be taxed at 28%.
If 28% of your income were withheld every week, they you'd have too much tax withheld, since the first $91K is taxed at lower rates. If they withheld taxes along the graduated scale, then you'd have very low withholdings at the beginning of the year, and higher withholdings at the end of the year.
Instead, the tax tables that your company uses to determine how much tax to withhold take your weekly income and extrapolate to predict what your taxable income will be for the entire year (your deductions are calculated from the number of exemptions you put on your W-4). That number is used to estimate your total tax bill for the year, then divided by 52 to determine how much tax is withheld for that week. If you earn significantly more in one week, you might have a higher percentage withheld since the extrapolation puts more of your income in the higher brackets. If your weekly pay was constant, then the percentage of tax withheld would be constant as well.
If 29-33% of your gross income is withheld, then it sounds like you have too much tax taken out, possibly because you have too few exemptions on your W-4. All that means is that you might have a large refund when you file your taxes.