Are the targeting-based restrictions for Twinned Spell metamagic and the Shield Master feat treated differently?

Question

After an erratum, Twinned Spell metamagic says:

To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level. For example, magic missile and scorching ray aren’t eligible, but ray of frost and chromatic orb are. [PHB, pg. 102]

So Twinned Spell metamagic cannot be used with an Area of Effect spell, even if it has only one target, because if someone else were present, they would also be targeted.

The Shield Master feat, on the other hand, says:

If you aren’t incapacitated, you can add your shield’s AC bonus to any Dexterity saving throw you make against a spell or other harmful effect that targets only you. [PHB, pg. 170]

Because this doesn't talk about the spell's ability to target multiple creatures, but rather only asks whether or not someone was the only one targeted, does that mean that a different interpretation applies? If someone is the only creature in the Area of Effect, does that count as them being the only target, even though the spell (or whatever it was) did target an entire area?

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AncientSwordMage has identified a relevant question: Which spells trigger the second benefit of Shield Master?

That discussion also references another related question: Does Shield Master only grant DEX save bonuses vs. single-target spells?

Answer 1

Yes, the ruling is different.

Twinned Spell states:

a spell must be incapable of targeting more than one creature at the spell’s current level.

So Twinned Spell is concerned not only with what the spell is doing right now, but also the full range of the spell's potential at this level. Even if a spell will target only one creature for the entire duration of its casting this time, if it is even capable of targeting more than one creature at this level, it cannot be twinned.

As you have observed, the language of Shield Master is different:

you can add your shield’s AC bonus to any Dexterity saving throw you make against a spell or other harmful effect that targets only you.

This does not mention anything about the spell's potential, so as long you are the only target at the moment, you may use Shield Master, even if there is the possibility that the effect can target more creatures at a different time or under different circumstances.

Note, area of effect spells target a point in space:

A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below).

So for example, a fireball that only harms you would not be an eligible effect, since it targets the point in space it erupts from.

That said, there are a lot of spells and a lot of different ways spells are worded, so I must leave you with the disclaimer that your mileage may vary from spell to spell and DM to DM. There are too many spells and too little consistency between them to tell you that this ruling can be trusted every time.

An example: acid splash

Acid splash states:

You hurl a bubble of acid. Choose one or two creatures you can see within range. If you choose two, they must be within 5 feet of each other. A target must succeed on a Dexterity saving throw or take 1d6 acid damage.

Since acid splash is capable of targeting two creatures, it is never eligible for Twinned Spell, but if its caster targets only one creature, and that creature has the Shield Master feat, that creature may use it on the saving throw for acid splash.