They are not the same. Later criticals on an arm with a shoulder hit will suffer additional penalties
Technically that is the same as how Hips work ;)
HIP (Page 48, HIP):
After a hip critical hit, ignore any other critical hit modifiers
from previous turns due to damage on that leg (even if this actually
reduces the PSR penalty provided by the leg). However, other critical
hits applied to that leg on this turn or later still apply.
Round 2; Foot Actuator
Round 3: Hip - Cancels Foot
Round 4: Upper Leg adds to Hip
So are you saying then a Shoulder is dentical in how penalites are applied.
I.E.:
OPTION A:
Round 2; Lower Arm Actuator (+1 Penalty to Hit)
Round 3: Shoulder - Cancels Lower Arm (+4 penalty to hit)
Round 4: Upper Arm adds to Shoulder (+5 total, +4 from Shoulder, +1 From Upper, Lower Arm Ignored)
or
OPTION B:
Round 2; Lower Arm Actuator (+1 Penalty to Hit)
Round 3: Shoulder - Cancels Lower Arm (+4 penalty to hit)
Round 4: Upper Arm does not to Shoulder (+4 from Shoulder, Upper and Lower arm ignored)