Form 1116 part 1 line 1a seems incorrect. How do I determine how TurboTax determined amounts on form?

The amount on Form 1116 Part 1 Line 1a is 'Gross income from sources within country shown above and of the type checked above (see instructions):'

This is the amount you entered when you entered your foreign source income, or when you entered your foreign taxes and were asked what income the tax was related to.

You can either go back and review your inputs, or print your return and look at the 'Form 1116 -- Foreign Tax Credit Comp Worksheet'.