Loding Tiff Image Vs PNG Images in QT

Hello,
I am developing application which handles and manipulates the images. While developing I have came across strange behavior with Tiff and other times of images.
In my app, if i manipulate and save the PNG, BMP, JPG images (like rotating/resizing), the saved images contains same bit depth as in the original images. But If i manipulate and save Tiff image (rotating), then saved image always have bit depth of 32.

Can anyone tell me why is this so??? Is there any way to save tiff image with original bit depth???

Thanks in advance

This may be a yet unresolved bug in Qt: http://bugreports.qt.nokia.com/browse/QTBUG-18475