Someone probably repaired the wall at that spot, or had a picture hanging
there for a long time, or spot-repainted.

I would only repaint part of a wall after warning my client that he WILL be
able to see the difference. I've even had problems finishing a can of
paint, then starting up the next day with a new can. (There's a reason the
manufacturer says to mix all the cans together before you start.)

Here's what I would do to fix your problem:

1) Clean the wall with TSP (Tri-Sodium Phosphate -- follow the safety
warnings on the label)

2) Sand the wall all over lightly to make the new paint stick better. Sand
a little more thoroughly around your problem areas.

3) Paint the whole wall.

4) Let it dry for 48 hours before using the shower.