It just seems a bit surprising that a person can accumulate money in a 401K without any of it being taxed on the front end, then withdraw a little at a time at retirement and pay no taxes on it then either. Is that right?
Yes, I think that is correct, it is one of the selling points for contributing to 401k's that most people are in lower tax brackets after they retire so they pay less (or no) tax on the deferred income.
Though now that I've retired and have been looking at discussions of it, turns out a lot of people put so much in their 401k's that when they reach 72 yrs and are forced to take the required minimum distributions, they are in a higher tax bracket.
Are you remembering to include your Social Security payments as income? They are sometimes taxable too, though they have a bizarre calculation, which luckily the dinkytown 1040 online calculator does automatically.
Here are the IRS instructions to determine if your social security payments are taxable:
Are Any of Your Benefits Taxable?
To find out whether any of your benefits shown on Forms SSA-1099 and RRB-1099 may be taxable, compare the base amount (explained later) for your filing status with the total of:
1. One-half of your benefits; plus
2. All your other income, including tax-exempt interest.
Exclusions. When making this comparison, don’t reduce
your other income by any exclusions
...
Figuring total income. To figure the total of one-half of your benefits plus your other income, use Worksheet A, discussed later. If the total is more than your base amount, part of your benefits may be taxable.
If you are married and file a joint return for 2021, you and your spouse must combine your incomes and your benefits to figure whether any of your combined benefits are taxable. Even if your spouse didn’t receive any benefits, you must add your spouse's income to yours to figure whether any of your benefits are taxable.
Base amount. Your base amount is:
• $25,000 if you are single, head of household, or qualifying widow(er);
• $25,000 if you are married filing separately and lived apart from your spouse for all of 2021;
• $32,000 if you are married filing jointly; or
• $0 if you are married filing separately and lived with your spouse at any time during 2021.