Yes, you are. Focusing is always done with the lens wide open. It's like said above, if more then one light up, then they all happen to be within focus.
A 50D can only use all (the camera pics the point/points to use) or one (manual selection). When the camera makes the choice, it will normally use the point(s) that see parts of the subject closest to you and where it finds anything to focus on.
The more advanced cameras (like 1D Mark III) allows you to expand focus to the neighboring points, but still not using all points. This can be useful if you are tracking something flying by, using Servo AF, at higher speeds. Keeping one certain point on the target may be difficult, and having them all enabled may confuse the camera into focusing on something un-related to what you are trying to catch on picture.