Buggy piston behaviour when other pistons are close by

When placing pistons horizontally adjacent but vertically one level apart, the upper piston won't retract until the lower one does.

In the case I tested this I used three pistons:

side view:

• - - - - -

• - - - - -
  r R S S S -
  S S S P P -

• - - P'S'-
  S S S S S -

top view:

r R S S S -

• - - R R -

• - - r r -

r = Redstone Wire

R = Repeater
S = Sandstone
S'= Sandstone with Repeater and Redstone Wire in front of it (see top view)
P = Sticky Piston
P'= Sticky Piston with Repeater and Redstone Wire in front of it (see top view)
T = Extension of an activated piston

• = Air

I then activated first the lower repeaters then the upper one:

• - - - - -

• - - S S -
  r R S P T -
  S S - T P -

• - - P'S'-
  S S S S S -

• - - S - -

• - - T S -
  r R S P T -
  S S - T P -

• - - P'S'-
  S S S S S -

The upper piston will now only retract if all repeaters are deactivated.