Two cooks measuring a teaspoon of salt into identical dishes can end up with markedly different seasoning. The spoon is the same; what fills it is not.
Crystal shape decides how much fits in a spoon
Fine table salt is made of small, regular cubes that pack together tightly with very little air between them.
Flake and coarse salts are made of large, irregular crystals that stack badly, leaving substantial empty space inside any given volume.
The result is that a spoon of fine salt can weigh close to twice what the same spoon of a flaky salt weighs, even though both are essentially the same compound.
Density varies within the same category
Salts sold as coarse or kosher are produced by different processes, some rolling the crystals into hollow flakes and others simply grinding them larger.
Those methods yield noticeably different densities, so two coarse salts are not interchangeable by volume either.
Additives compound the problem, since anti-caking agents and added iodine slightly change both the packing and the perceived flavour.
How the spoon is filled matters as much
A spoon dipped into a container and levelled holds more than one loosely scooped, and salt taken by hand is not measured at all.
Compaction from tapping or from the salt settling in storage adds further variation that no recipe can account for.
Weight avoids all of this, which is why recipes written for accuracy give salt in grams and note which type was used for testing.
Perception adds a second layer of difference
Large flakes sitting on a finished dish dissolve on the tongue and register as sharp bursts, while the same weight dissolved evenly reads as milder.
That is why a finishing salt is chosen for its texture and why substituting fine salt at the table produces a saltier-tasting result from less material.
Salt dissolved in cooking is a different matter entirely, since once in solution the crystal size no longer exists.
What this means for following a recipe
Where a recipe states a salt type, matching it is worth more than matching the quantity, because the quantity was calibrated to that crystal.
Where it does not, salting in stages and tasting is the only reliable route, since the written number carries less information than it appears to.
Baking and curing are the exceptions where guessing is not available, and those are precisely the recipes that most often specify a weight.