Numeric grades exported in the Excel/XLS file format from the various locations in Gradebook are represented in Excel as text. This prevents formulas from running as expected. In Excel this problem is indicated by the numbers being aligned to the left, not to the right as numbers are when numbers are added to cells in Excel itself.
There are two work arounds:
1. use the CSV file and import it
2. convert the text back to numbers in Excel, provided that it's a version after Office 2002
This is not unique to Sakai, and happens with a lot of tools that import/export content to Excel: http://www.dailydoseofexcel.com/archives/2006/02/18/number-stored-as-text/
It would be nice to have numbers in this Excel file created as numbers, otherwise there's no real advantage to having an Excel option. Further to that, we'd be happy with a sakai.properties option to surpress the Excel/XLS version.