Net Investment Income Tax is just one of the the things that could explain your marginal tax rate being higher than your tax-bracket rate, but is a good guess given the result. There are all sorts of other reasons that the increase in your AGI caused by the additional income could result in an increase the taxable amount of some other income or decrease the amount of some tax credit, but those would be unlikely to cause your marginal tax rate to be exactly 3.8% higher than your tax-bracket rate.
