The ten percent that you had withheld is probably a lower percentage than your marginal federal tax rate. The tax rates rise as your income rises, from a low of 0 to 12%, then 24%, then 32% on up to a maximum federal tax rate of 37%. So, you are getting credit for the taxes you had withheld, but it was not enough to cover the marginal tax on your pension distribution.
Here's a link to the federal tax brackets:
https://www.kiplinger.com/taxes/tax-brackets/602222/what-are-the-income-tax-brackets-for-2021-vs-202...
For the state tax, you probably didn't have enough withheld either, but you'd have to look up the tax rate in your state to check on that.
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