What the number is actually telling you
A low slump means a stiff load that barely settles when the cone comes off. A high slump means a loose, wet load that spreads out further. Neither end of that range is automatically wrong, since the right slump depends entirely on what the concrete has to do once it leaves the truck. What a slump number does not tell you is strength. Two loads can test the same strength at very different slumps, because slump is about how the mix behaves fresh, not what it becomes once it cures.
Why the right number changes from job to job
A slab poured into open forms with a chute reaching straight to the spot needs a different slump than a load traveling through 2.5 or 3 inch line under pressure. Too stiff and a load resists moving into forms cleanly or packs against the inside of a hose. Too loose and it can separate, with the stone settling out and leaving soup at the surface. The correct number for a given job comes out of the plant's mix design for the strength and exposure ordered, which is why we do not hand a single figure to every job. We tell the plant what the concrete is for and how it is being placed, and their mix design lands the slump in the workable middle.
What it looks like when a load is off
A crew can usually tell within a minute of the chute opening whether a load is where it should be. A stiff load fights the vibrator and drags in the forms. A loose one runs too far and leaves the crew chasing it. Either way, the fix is a phone call to the plant about that particular truck, since suppliers have real corrections for a stiff load that do not touch the concrete's strength. Adding water at the tailgate is not one of those corrections. It buys workability by spending strength that the slab never gets back.
Why slump matters more once a pump is involved
A load headed through 2.5 or 3 inch line has narrower margins than one going straight down a chute into open forms, since the same mix that places fine into open forms can pack a line if it runs too stiff. We cover the full list of what a mix needs to travel clean through hose, slump included, on our page about pump mix requirements.