I think I see the confusion here. These forms can make one's head hurt.
The 8960 only winds up taxing investment income. It never taxes wages or social security.
What line 13/14/15 are doing is figuring out whether or not the NII even applies. That calculation is based on all of your income not just investment income! Here's a screen shot from a test case with a lot of wage income but only $100 of investment income. The NII tax is only $4 ...
It isn't about what the IRS is trying to do. It is about how Congress wrote the law. See I.R.C. 1411(a)(1) and how the 8960 exactly maps to it.
https://www.law.cornell.edu/uscode/text/26/1411 ...
you can see in 1441(a)(B)(i) it is your total income (adjusted a bit) not just your investment income. However you can't be taxed more than your NII.
From the statute:
In the case of an individual, there is hereby imposed (in addition to any other tax imposed by this subtitle) for each taxable year a tax equal to 3.8 percent of the lesser of—
(A) net investment income for such taxable year, or
(B) the excess (if any) of—
(i)the modified adjusted gross income for such taxable year, over
(ii)the threshold amount.
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