That sounds like the data are somehow being imported in an an incorrect format. Numbers stored as text is one possibility that has already been mentioned. Or numbers with incorrect decimal and/or thousands separators, preventing them from being recognized as numbers. Or numbers with extra spaces or other non-printing character that prevents Excel from seeing them as numbers.

Debugging steps I would start with

Use ISTEXT() and/or ISNUMBER() to see if the data are being recognized as text or numbers
Use the TRIM() function to remove unwanted spaces/tabs/nonprinting characters.

See where that gets you.