I took some pictures of 2 people in a room with a window behind them. I focused on the most important subject, kept the shutter button half-way down, then recomposed so both of them were in the frame, putting the focus point between them. They were moving a little bit so the focus point at the time I took the exposures was sometimes on a wall and sometimes on the window glass, with the light coming in from outside. I assumed that the exposure was set when the focus was set but it wasn't. In the exposures where the focus point was on the wall, the exposures aren't great but they're okay. When the focus point was on the window at the time of exposure, the window light caused the exposure to go way down so the subjects were silhouetted.
Is this normal behavior or have I caused this by setting something off the default?
I'm using a Rebel XSi in aperture-priority mode and spot-metering.