Holding the charge of instantaneous duration touch spells

[poiop]

How can a caster, like a Magus, hold the charge of a touch spell, like Shocking Grasp, if its duration is "instantaneous"

Duration: instantaneous:

The spell energy comes and goes the instant the spell is cast, though the consequences might be long-lasting.

Touch Spells and Holding the Charge:

In most cases, if you don't discharge a touch spell on the round you cast it, you can hold the charge (postpone the discharge of the spell) indefinitely. You can make touch attacks round after round until the spell is discharged.

If the energy of Shocking Grasp comes and goes in a single instant how can the caster hold the charge? It says "in most cases,..., you can hold the charge" may this case be an example of situation in which it's impossible to hold the charge, or not?
I know the "consequences" can be long-lasting, but I think this means only that the consequences of the shock on the enemies, so the damage, remain.

[Lady Asharah]

Instantaneous duration only means that the effect cannot be dispelled and doesn't require concentration to maintain (so you don't roll concentration checks if you are taking damage while holding a charge)

[blahpers]

A touch spell's charge is a delivery mechanism, not a direct effect of the spell. Duration applies to the effect of the spell being applied to its target, not to the charge.

Charge can generally be held indefinitely so long as the caster doesn't touch anything. This is true even for spells like touch of fatigue that have a finite duration. The duration (if any) doesn't kick in until the caster touches something, and then only if it's a valid target. (If it isn't a valid target, the spell discharges harmlessly.)