Why federal due bumps after importing foreign income which can be excluded

[jicongrui]

We have about 400K US income and 100K foreign income from China which could be excluded.

After importing the foreign income, the federal due bumps ~20K, could someone help to tell the reason?

[maglib]

If you claim the foreign earned income exclusion, the housing exclusion, or both, you must figure the tax on your nonexcluded income using the tax rates that would have applied had you not claimed the exclusions. See the Instructions for Form 1040 and complete the Foreign Earned Income Worksheet to figure the amount of tax to enter on Form 1040, line 11a. If you must attach Form 6251 to your return, use the Foreign Earned Income Tax Worksheet provided in the Instructions for Form 6251.  Turbotax does this calculation seamlessly for you.  Does that make sense?