It is a known issue that Excel sometimes only detects some fields as numbers. Read more at http://support.microsoft.com/kb/181298.
There are multiple ways to solve this:
- Solution 1
- Use the CSV export instead, and during import you can map the columns to the correct type (if not detected automatically).
- Solution 2
- Select all the data (CTRL-A)
- In the top-left corner you will now see the exclamation mark symbol.
- Clicking the symbol will give you the option, "Convert to Number".
That should do the trick!