Every plan gives you dimensions. Not every plan tells you where they came from.
A dado is 1/4" deep in 3/4" stock. A panel floats 1/8" inside its grooves. A tenon is 2" long and 1/3 the width of the rail. Those numbers are all correct. They're also opaque: the plan tells you what was decided, not why, or what happens if you need to decide differently.
That missing reasoning is the gap. It's in every plan you'll ever buy, download, or borrow from a friend. It's not a flaw. It's just what happens when someone who knows the answer writes it down without showing their work.
Three kinds of gap
Structural gaps come from physics. A dado at 1/3 the panel thickness is holding a shelf across a 30" span. The depth isn't decorative. Make it shallower, and at some point you run out of floor to carry the load. The number came from a calculation, even if the designer did it in their head twenty years ago and stopped thinking about it.
Movement gaps come from wood being wood. A frame-and-panel assembly with 1/8" of clearance between panel edge and groove bottom exists because flat-sawn white oak in a humid climate moves a predictable amount across a predictable humidity swing. The 1/8" isn't generous or tight in the abstract. It's generous or tight relative to the species, the cut, the panel width, and the shop it lives in.
Knowledge gaps are the biggest category and the hardest to spot. They're the places where the plan assumes you already know something the drawing doesn't say. Which face is the reference face. Whether the measurement runs to the shoulder or the outside of the tenon. Which surface gets machined first. A designer who knows the answer doesn't explain the answer. You either bring that knowledge to the bench or you discover you needed it after the first cut.
How to find the reasoning
Ask "why" at every dimension that doesn't explain itself.
That 1/4" dado depth. Why not 3/8"? In 3/4" stock, 3/8" is half the thickness. For a light shelf on a short span, that's fine. But 1/4" is one-third, and it leaves more material in the panel. If the plan chose 1/4" and the span is 30" under a load of books, the depth is doing structural work. If the plan doesn't mention the span on the same page as the dado depth, you have to connect those two numbers yourself. That connection is the gap.
The frame-and-panel clearance is the same problem from the movement side. The plan says 1/8". Your shop is in Penticton, not the humid climate that clearance assumes. Your humidity swing is different. Your species might be different. Is 1/8" still the right number? It depends on whether the designer's movement math matches your conditions. The plan can't tell you that because the plan doesn't know where you are.
Why this matters more than the number
If you understand why a dimension is what it is, you can change it when the situation changes. If you only know the number, you can only copy it.
Swap species, and every movement-driven dimension needs recalculating. Swap tools, and every tolerance-sensitive operation needs evaluating. Change the span, and every structural ratio needs checking. The plan designed around cherry, built with the designer's cherry, in the designer's shop, will behave the way they expected. Your cherry, your shop, your humidity range. The dimensions are the same. The behavior might not be.
The gap is where the plan becomes yours. The builder who asks "why is this number what it is" before reaching for a saw is making the reasoning visible. Once it's visible, you can evaluate it. Once you can evaluate it, you can adapt. And adapting is the whole point. Nobody builds the same piece twice in the same shop with the same wood.
The plan is an argument written in numbers. Learn to read the argument, not just the numbers.