First thing, I made a typo. I said parry gives -4 but really it would be -5 with the shield (Melee skill +1 is a negative to the attack being received)
Why would you get your full Melee skill while using a shield? Because that's how parries work per RAW. Core page 76 states:
"A Traveller in close combat may attempt to parry an
opponent’s melee attack as a Reaction. In so doing,
they will inflict their Melee skill as a negative DM to the
attacker’s attack roll."
It does not specify a specialty under Melee, it just says Melee. That and common sense would suggest that any of the specialties you have learned comes with a parry ability that could translate into defense moves that could parry any of type of melee attack. Being that as a ref I tend to err on the side of defense, I tell my players they can simply use their highest Melee skill. Though this does not contradict RAW, I admit they could have been clearer about it and stated as such.