In my case, I think the reason was because of my overpayment of FICA/social security. I switched main jobs in September, and had just hit the maximum FICA contribution before that. So in my new job, all of the FICA I paid was overpayment, which TurboTax automatically credits toward your federal taxes. So my Federal tax payment number was equal to my federal tax paid plus my FICA overpayment.
It's all moot in my case at this point, as I am planning to contest the penalty via a first time penalty abatement waiver - not something that the IRS or TurboTax advertises. The YouTube video below discusses this, but take it with a grain of salt, as it is just a YouTube video. Also, I am not a tax attorney, so take my strategies with a grain of salt as well.
https://www.youtube.com/watch?v=jOZ1RAkhpD0&t=267s
