Outlook does not establish which program to open a file with. It simply references whatever Windows has set for the default application. I would ensure that your default program is indeed the version of Excel you want. Your computer may have changed the default to 2013 once you installed it since it is the newest.
To change the default program, right click on one of your excel files with the extension you want to change, click open with.. click choose default program.. Check "Always use the selected program to open this kind of file", select the correct Excel version, and then click ok and it should be set.