I went through the same puzzling exercise, trying to find the source for line 2a through the magnifying glass and related info. The number in line 2a doesn't show up anywhere else in the print-for-records PDF forms. Finally checked here as well as IRS instructions and found the answer that its a combination of several lines from 1099-INT and 1099-DIV as indicated.
Strong suggestion for TurboTax writers: create a worksheet that shows this addition/subtraction process with a line for each item of source data so it's traceable and include in the print-for-records PDF.
