En complément du point 6, l'explication de Chuck Westfall , Canon USA

1. In the case of EOS SLRs with less than 45 focusing points, the AF frames are engraved directly on the focusing screen. In some cases, the focusing screen may become very slightly shifted to the left or right of the actual focusing point position.
2. Also in the case of EOS SLRs with less than 45 focusing points, the size of the AF frames on the focusing screen is smaller than the size of the actual focusing points. If there is more than one subject in the area being analyzed, the camera may pick the area with the highest contrast, and this area may be slightly outside the AF frame in the viewfinder. However, under no circumstances does an EOS SLR with less than 45 focusing points and set for manual focusing point selection evaluate another focusing point than the one manually selected, unless the camera supports focusing point expansion and the user has selected it.
With EOS-1D class cameras including the EOS-1Ds Mark II, the size of the actual focusing points is very close to the size of the AF frames superimposed on the focusing screen, and moreover the alignment of the AF frames shown in the viewfinder compared to the positioning of the actual focusing points is typically quite accurate