From what I've seen in Schwab and experienced personally. The "sell shares for taxes" settings is a flat tax rate, mine is 26% last I looked. If you're in the higher brackets[1], ie making 250k+ it can be up to 9% difference. So if you get a vest of 20k in November toward the end of the year where you're likely in that higher bracket the amount Schwab sells could be 1,200~ difference (assuming 32% tax bracket, 26% flat), meaning you'd owe the federal gov that come tax time.

If you speak with a tax professional, for which I am not, they would tell you to calculate the difference and pay quarterly amounts. In practice this means that I sell more periodically just for taxes[2].

1. https://www.irs.gov/filing/federal-income-tax-rates-and-brac... 2. https://www.irs.gov/businesses/small-businesses-self-employe...

Edit: from what I understand payroll isn't informing Schwab of your current bracket, nor do I think they should have to.