Why do I have to pay additional medicare Tax-$115.00 per turbo in Form 8959? Medicare Taxes were already withheld per W2s(3)total medicare wages $262,763.00?

Form 8959 has a maximum amount $250,000.for joint filers. What does this mean? difference $12763.00 is multiplied by .009 instead .0145? Why I'm now considered as self employed?