When iterating through the rows and cells of an imported Worksheet in at least one of the rows GetValue().Value.ResultValueType for a number cell is set to GetValue().Value.ResultValueType.Text and the value to "error" or "string".
For us this happens in a worksheet with 200 rows max. and columns formated as date, time, time followed by some text and number columns.
In our Tests with different excel files it allways happened once (between line 11 and 16) and only for the first two columns.
Exporting the worksheet back into a new excel file will show all rows and columns types and values OK.