Why is Box 3 on the 1099-INT from Treasury Direct taxed at a 42% rate?

TurboTax shows a federal tax due of $675.

When I manually enter a 1099-INT from Treasury Direct, where Box 3 reports $2,460, the federal tax due increases to $1,705.

Is my calculation correct?

(1705 - 675) / 2460 = 41.87%

 

Does that seem right?

 

My total income is in the $243,726 - $609,350 range (Married Filing Jointly).