It’s often the case that you sell a block of RSU stock, and the broker reports the entire sale as a capital gain (e.g. if you sell $50K, then it shows up as a $50K capital gain on your broker’s 1099). But in reality, the gain should only be the difference between the price it was when it showed up in your account and when you sold it (which can actually be a loss), because the basis is already included in your W-2 as earned income.

E-Trade and Fidelity report gains correctly. Which broker doesn't?

My recollection is they report it on a “supplemental information” form, not directly on the 1099. If you just do a naive import in TurboTax or similar, you’ll overpay your taxes by thousands.

The IRS requires your broker to report a different cost basis to you than you are entitled to use. It is government-mandated disinformation, not a misunderstanding.