Why only the two end risers move
A middle riser is measured tread to tread. Put the same thickness of covering on both treads and it cancels: the top of one goes up by the same amount as the top of the next, and the riser between them is unchanged. That is why recovering a stair does not, on its own, change the middle of the flight at all.
The two ends are different because each one has a landing at one end and a tread at the other. The bottom riser runs from the lower floor to the first tread, so it shrinks by whatever the lower floor gains. The top riser runs from the last tread to the upper floor, so it grows by whatever the upper floor gains. Change one landing and not the other and the flight ends up with one riser that does not match, at the end where somebody has stopped counting.
Taking the old covering off is half the answer
The number that matters is the net change, not the thickness of the new floor. Carpet and a pad together can be more than half an inch, so pulling them and laying a rigid plank of the same thickness moves nothing at all. Going the other way — carpet over what used to be vinyl — adds the lot. The page asks for both figures at each of the three places for that reason, and a job that looks like it adds half an inch everywhere often turns out to add nothing anywhere.
The variation figure is not this page to give
How much the risers in one flight are allowed to differ is set by the code the local authority has adopted, it is enforced by whoever inspects the work, and it is neither the same everywhere nor the same across years. So it is a field on the form. Put in the figure that applies where you are, and read the comparison as arithmetic rather than as a verdict: this page cannot approve a stair and does not try.
Everything else that loses the same half inch
Doors are the obvious one, and they have their own limits — a hollow core door has a thin solid rail at the bottom, and a rated door is not something to trim at all. Less obvious: the opening a dishwasher slides into, which is measured from the underside of the counter and does not grow back; the toe kick under a run of cabinets; floor register boots that end up below the finished surface; and the bottom of every door casing, which has to be undercut so the new floor runs under it rather than being scribed around it.
Questions people ask
Why is my top step shorter after new flooring?
Because the upper landing got thicker and the tread below it did not. The top riser is measured from the last tread up to the finished upper floor, so every fraction added up there comes straight out of that one riser while the rest of the flight stays where it was. It is the single most common way a consistent flight of stairs ends up with one odd step.
Does recovering the stair treads fix it?
It moves the problem rather than removing it. Covering the treads cancels out of every riser between them, but it lifts the first tread relative to the lower floor and lifts the last tread relative to the upper floor, so the bottom riser grows and the top riser shrinks by that thickness. Matching the tread thickness to the landing thickness at both ends is what actually keeps the flight even.
How much can risers differ?
That comes from the code the local authority has adopted and from the inspector who applies it, and it is a field on this form rather than a number printed here. It is worth finding out before the material is ordered, because the remedy afterwards is carpentry — rebuilding a tread or a landing — rather than anything you can do with flooring.
Will my doors still clear the new floor?
Enter the gap you have now and the page subtracts the net change at the lower floor. If the answer is negative the door has to come down, and how much can come off is the door manufacturer figure rather than a general one: a hollow core door has a narrow solid rail at the bottom and the skins need something to hold on to. A fire-rated door assembly should not be cut.
Will the dishwasher still come out?
Measure from the underside of the counter to the existing floor and put it in the appliance field. The counter does not move, so every fraction the floor gains comes out of that opening, and a machine that went in on adjustable feet at full extension may not come out again over a thicker floor. The usual answers are to stop the new floor at the front of the cabinet or to lift the counter, and both are decisions made before the floor goes down rather than after.