Aditya,

The new canon cameras also have auto ISO. The ISO is increased depending upon the situation. However, I don't know why it would underexpose the scene.

To start with it might be a metering issue. If you provide the correct expsoure compensation, then I think it should be ok.

Pulling up the exposure later will increase the noise in the shadows. Please check this link:
http://www.indiawilds.com/forums/showthread.php?t=2006