This is your terminal not using its full allocation, probably because it violates ICCCM and considers its space request to be obligatory on the window manager.

http://xmonad.org/xmonad-docs/xmonad-contrib/XMonad-Layout-LayoutHints.html *might* help, but you may need to consider using a non-broken terminal program.  Too many lazy X11 programmers don't understand that the window manager has the final word on window sizes and it's up to their program to deal.