A recipe that works reliably for its author can fail repeatedly for a reader who followed it exactly. The instructions carry hidden assumptions about equipment that are almost never stated.
Heat output varies enormously between stoves
Burners differ several-fold in power, so a medium setting on one hob delivers what a high setting delivers on another.
Instructions written as a dial position are therefore meaningless outside the author's kitchen, while instructions written as an observation remain valid anywhere.
This is why well-written recipes say to cook until the onions are translucent rather than to cook for six minutes, and why the ones that only give a time cause the most trouble.
Pan size changes the outcome more than the timing
The same quantity of food spread across a wide pan has far more surface exposed to heat and air than the same quantity in a narrow one.
A wide pan evaporates liquid quickly and browns readily, while a crowded narrow pan holds moisture and steams whatever is in it.
Since recipes rarely specify a diameter, two readers can follow identical steps and produce a reduced sauce and a watery one respectively.
Ovens are inaccurate and inconsistent
Domestic ovens commonly run above or below their set temperature, and they cycle around that point rather than holding it steady.
Fan and conventional settings also cook differently at the same nominal temperature, because moving air transfers heat much faster than still air.
An oven thermometer resolves the offset, and once known it can be applied to every recipe rather than being rediscovered with each one.
Ingredients themselves are not standardised
Flour protein content varies by brand and region, so the same weight absorbs different amounts of water and produces a different dough consistency.
Onions, tomatoes and vegetables vary in water content by season, which changes how long a base takes to reduce and how much liquid a dish ends up with.
Recipes are written against whatever the author had available, and the further a reader's supply is from that, the more adjustment is required.
Altitude and water change the physics
Water boils below its usual temperature at altitude, so everything cooked in liquid takes longer and no amount of extra heat changes that ceiling.
Hard water slows the softening of pulses and affects doughs and infusions, which is why the same dal recipe behaves differently in two cities.
None of these variables appear in the method, so a recipe is best read as a description of what should be happening rather than as a set of instructions to be obeyed.