Pg 161 of CRB:
Missiles are launched in salvos. A salvo is all the missiles launched by a ship against a single target in the same combat round. This could be a single missile from one turret or dozens from multiple turrets or bays (see High Guard for more information on weapon bays).
Pg 162 of CRB:
Variant Missiles - High Guard introduces different types if missiles that are more accurate, carry more fuel or are faster, but these rules suit all missiles included in this Core Rulebook. If a ship launches different types of missile at the same target in the same round, then all the missiles of each type are counted as a different salvo.
Under this scenario, then torpedoes, or even a variation on a standard missile would be counted as a separate salvo. While you would lose your positive DM by grouping missiles up, you would also force your opponent to choose which missile, or torpedo salvo they are going to try to use EW on. Say a 400 ton ship had four triple missile turrets firing four different types of missiles, that would count as four separate salvo's, correct?
And from a point defense perspective, pg 160 of CRB
The gunner must succeed at a Gunner (turret) check against any missile salvo that is about to make its attack roll against his spacecraft. The Effect of the check will remove that many missiles from the salvo. A double turret equipped with lasers provides DM+1 to this check, while a triple turret will provide DM+2
Using our example above, the attacking player launches four salvo's of three missiles each. The defending player can engage one salvo per turn with EW, and when the salvo arrives at his ship he can engage one salvo per turret he has set aside for defense.
I can see players mixing up expensive specialized missiles with standard ones, kind of like Stratego - you don't know what's going to hit you till it hits you and does damage, and mixing up your missile types will force the defending player to engage different types of missiles - he best hope he gets the deadlier ones first.
As an aside, the description on variant missiles should have been grouped with the missile explanation. Page 162 is changing what was defined on page 161.