Treasury Bill Interest - 1099-INT Box 3

Hi, All.

 

Entering treasury interest in Box 3 of the 1099-INT should be subject to my marginal federal tax rate, right?  Let's say I am well-within the 24% bracket where the added treasury interest does not come close to the next bracket, why would TT calculate a higher rate (in my example case: 27.8%), instead?

 

Any insight is appreciated.  Thanks in advance.