I may have solved the problem by changing the focusing point to a single point rather than the 7 points that is the regular way. In the idiot modes, you don't have the choice of focus points thus you get many pictures that are too dark. I am concerned that the metering system is not working properly...
You've just answered your own question, the DRebel (like the 10D) will biais its metering toward an AF point that the camera decided to pick (the one that lighted up). If this AF point landed on a bright area you'll get an underexposure, and if it happened to be a dark spot you'llend up with an overexposure instead... It just how the camera works.
You can also press the AE *
lock button to force the camera to meter the cenral area only when in the "idiot" modes or creative zones ...