We've recently upgraded to Excel 2013 and users are finding an issue with the decimal place format when adding up a number of cells with percentages in them. We set the percent format to 2 decimal places but when the field calculates, it blows out the setting to 17 decimal places. I have all the latest service packs installed and have tested it on a variety of workstations in-house using an existing Excel file with this formula in it or creating a new spreadsheet as a test...we can make this problem happen. Of the 3 places you can control the format of the decimal place (\Options\Advanced, cell format or conditional formatting) all have been verified as not being the cause of the issue.
Here's the formula that results in the blown out decimal places:
Each of these cells has a formula in it that is calculating a number which is formatted as a percent.