You can change the Advanced setting that automatically converts A4 to Letter in Microsoft Office Excel.

1. Open Microsoft Office Excel.
2. Click the Office button.
3. Click Excel Options.
4. Click Advanced.
5. Scroll down to the General section.
6. Clear the Scale content for A4 or 8.5x11" paper sizes option.
7. Click OK in the Excel Options window.
8. Restart Microsoft Office Excel.