I see that the performance based stocks granted to me in year 2019 (say 100 stocks), in addition to the yearly compensation, say $100K, are being reflected as wages in my W2 . Let's say that the total value of the stocks is 100*50 = $5K. My taxes are therefore calculated on the basis of my wages (which is $100K + $5K), meaning that the stocks are being taxed as part of my yearly income. So far so good.
I also, however, see on the broker website (that maintains the above company granted stocks) that even though the number of 'vested' stocks is 100, the 'sellable' quantity is only 60. This means that the broker is also pre-taxing the total number of stocks granted. Also, I do not sell any stocks for the year 2019.
Why is the broker pre-taxing the stocks when W2 is already reporting them as income? Am I not already paying taxes on my stocks as part of my yearly tax return? Is this not a case of doubly taxing the same income? I'm pretty sure I'm missing something here, but not sure what.