The focus sensors aren't merely tiny points. Check out this
picture that shows the 10D's viewfinder overlay and the "actual"
focus sensors.
http://www.photo.net/bboard/uploaded-file?bboard_upload_id=14972084
Note that the 10D viewfinder has fairly substantial boxes to
indicate the focus point wherease the 300D viewfinder uses tiny
dots. In either case, you can see that the actual sensors are
quite a lot larger than what is indicated in the viewfinder.
This means two things:
(1) If you happen to focus on something that takes up a very small
part of the view, you can easily miss your target.
(2) If you focus on something that isn't flat perpendicular to your
camera (i.e.: both your examples were sloping away from the
camera), you are likely to get focus for the closest part of the
sensor guaranteeing a slight focus miss! Yikes! (And, with a
relatively small depth of field, that could spell real trouble.)
Number (2) might be the problem with your map picture. (I can't
see the map in enough detail to determine anything.) Let me
guess... the actual focus of the map is closer than where you were
aiming? On the 10D camera I would set to use only the bottom focus
point (which is horizontal in nature), set single-focus mode,
half-press the fire button to focus on my intended target with the
single bottom focus point, recompose the picture, and then fire.
Here's another interesting read about the focus issues:
http://emedia.leeward.hawaii.edu/frary/canon_eos10d_03.htm