It didn't add the sum of interests. It added the sum of the two principle values for loans held at different times. And then claimed this was the average outstanding loan. It's off by a factor of 2.
The law says I can deduct interest on up to $750,000 of my mortgage. At an average of 3%, that would be $22,500 per year. But Turbo Tax computed around $10,000, because it divided 750000/(sum of all loans) rather than 750000/(average loan held in any month).
This is a huge difference and a big error.
