They are probably text-formatted if a text function has created them as in:

=LEFT(A1,2)

while A1 houses something like 215 or 21A.

=--LEFT(A1,2)

or

=LEFT(A1,2)+0

would make the result a true number.

Veronique wrote:
I tried to add a range of cells which values came from a
function and the result was 0. It didn't seem to
understand the result of my function as a true number.

any hint on how to add this range of cells?

Thanks much,