The other day in game, we had someone throw a Missile spell right near 1/2D range. That surprised some of my players - why do Missile spells have half damage ranges? Especially explosive missile spells?
My short answer was, because they do, and they always have, and GURPS has a history of making spells effectively less powerful the further out you go (be it by effect or by difficulty to cast.) Even if it didn't though, the spells says it has a half damage range, so it does.
I mean, "logically," a thrown missile spell shouldn't dissipate, because it's magic. But equally logically, a missile spell should, because that's how magic works. Logically, a magically created fireball should have issues with a No Mana Zone, except it doesn't, because just as logically it's a physical manifestation of a spell and there isn't any "magic" to the missile, just the creation and launching.
But it does A not B, C not D, E not F, because it's magic and the spell says that's what happens.
That's kind of the point of magic.
It's magic, not science.
First and foremost, spells are magic, not science. You have to look at them through the lens of "and then a miracle occurs" as the middle step between "I cast [some spell]" and "[this thing happens]." It only has to follow physical laws if the spell invokes them. No Second Law of Thermodynamics has to hold back spells that create or destroy matter unless the spell says so. You don't need to consider drag of an subject with the Flight spell on it, just encumbrance carried by the subject, because magic. You can be Invisible and that's that, no "but wouldn't I be unable to see if light is bent around me? Wouldn't robots see me if it was clouding minds? Wouldn't . . . " - no it wouldn't. The spell just makes you invisible, and doesn't blind you, cloud minds, affect light in any strange way, etc. It just does the thing.
That thing may have physical and scientific consequences (Earth to Air the support beams, the ceiling might fall, a Fireball might start fires), but it might not (Create Earth doesn't summon earth from somewhere else to obey a physical law, a Teleport spell doesn't cause an implosion of air rushing in to the spot you left.)
It's internally consistent, not externally consistent.
Spells follow their own logic, not an overall system of logic. For example, the Missile Shield spell averts any missile from hitting you by some small margin. So if I throw an axe at you from one yard away, it misses, because that's what Missile Shield does. If I swing it from one yard away, it does nothing, because it's not an attack covered by the spell. Punch you with a rock in my fist? Not affected. Throw it or drop it? It's covered.
But the Bladeturning spell works regardless of it's a missile or not. But equally, if it's an impaling tip, it doesn't turn that. or even a spear with a cutting edge. Missile or not, because that's how the spell functions. It doesn't matter if that's how another spell does it.
You could tidy up the spells by making them work consistently with each other, but as long as the spell itself works with the wording it's got, and it has consistent effects, it's good. That Sunbolt creates a missile that can't be turned with Missile Shield is totally fine; the spell has its own internal logic and then meets the internal logic of the other spell and wins out.
It works how it works.
Spells do what they say, not more or less. Well, possibly less, but not really more. "But doesn't that mean . . . " is the start of an exercise in creativity, for sure, but it also is a way to expand a small investment in points into a catch-all spell that does everything. The old "Why can't I create air pockets in someone's veins with Create Air or destroy the oxygen in their brain with Destroy Air?" thing is cute, but the spells don't say they do that. There may be spells for that, but you'll need those spells.
This is not to say that you couldn't have a magic system with scientific underpinnings, or which has a consistent explanation across all fields (no Missile Shield spell, only a "weapons between the sizes of X and Y miss automatically," say, or wishing spells affect probability only, or creation spells that actually move matter around but don't create it.) But that's not necessarily the system at hand. It's not the system I run, either. Science and logical consistency across spells and effects is for figuring out edge cases or where specifically invoked. It's not a trump to how the spells are written. And "It's magic" really does count as a valid explanation of why something occurs in a magic system.