When my sheet is unprotected, for some cells, a green mark on the left-top corner appears with the following message.

« the number in the cell is formatted as text or preceded by an apostrophe ».

I don't understand that thing because all of the cells have a numeric format.
I have tried to reformat that cells programmatically... but no effect.
First, what is the reason (copy and paste)?
Second, is it possible to replace all that cells to convert to numeric programmatically?