TOAD will incorrectly auto map columns when importing an Excel file from the following menu:
Database > Import > Table Data
The problem occurs when importing Excel spreadsheets that have several workbooks. The first workbook will always limit the number of imported columns for the other workbooks. So say if workbook 1 has 4 columns, and workbook 2 has 6 columns, then when the auto mapping import is executed, workbook 2 will only have 4 columns imported.
Waiting for fix in a future release of Toad.