I figured it out, it's the HTML tags "<TABLE> <TR> <TD>" that cause Excel to format the file incorrectly. I removed them and it works fine now.

Now I have another problem: the product description is truncated when opened with Excel. The maximum number of characters in each cell is 255. Most of my product description has longer length than that. How do I lift this limit? With this limit, the whole purpose of using Easy Populate is defeated because I cannot update product information with truncated description!

(I know my file lacks the product model, but I can work on it later.)