Paint Question: Why does this happen? (best, wall, area, new)

[gizmobizmo]

I'm no expert on paint by any means but this one has me stumped.

I usually paint the walls of my houses using a Satin or Eggshell. I always heard that is best if you need to wash off a wall if it gets dirty (pets or kids have that effect).

At a later date, when I go back and touch up an area with the same paint, that area has a different sheen to it and from that day forward. I don't get it. It's the same paint. Why does that happen?
Any ideas to consider for future paint projects? I hear flat paint is best.

The walls on my new house are a little scuffed from when the previous owner moved out and now it looks like I'm going to have to re-paint the whole wall just so it looks nice and neat instead of polka-dotted. ***sigh***

[the painter]

It is all about sheen. Touch up on a higher sheen is difficult, it is best to repaint the whole wall. Also paint "ages" so if the walls were painted say 6 months ago even a flat paint will show touch up spots at times, even if out of the same can.

On the higher sheen try using just the tip of the brush and dab, dont use a full brush stroke or roller.

[sgresident]

I hate this! I have the same issues. try a flat paint.

[Wysiwyg]

maybe you need to wash it too to match the older paint