The mockup got sign-off. Everyone in the room agreed it looked right. Three weeks later the live page has every element the mockup had, technically, and it still feels a little off. Nobody can quite say why, so it ships anyway.
That gap has a name, and it isn't the developer's fault, whatever the meeting where it gets discussed tends to imply. It's a handoff problem, and it happens in a fairly predictable place.
Where it actually breaks downThe static file was never the whole spec
A design file shows one state, at one width, at one moment. It doesn't show what happens on a slow connection, what the hover state looks like, how the spacing behaves at 1400 pixels versus 375, or what the section looks like with a headline twice as long as the placeholder text. All of that gets decided during the build, by whoever's building it, under whatever time pressure that week happens to have. Not because they're guessing carelessly. Because nobody wrote it down.
The second gapReview happens on desktop, once, near the end
By the time anyone reviews the live page properly, it's usually close to launch, on one screen size, and the cost of changing anything has gone up considerably. A spacing issue that would have taken ten minutes to fix during the build takes an afternoon to fix after everything downstream has been built around it. So it doesn't get fixed. It ships slightly wrong and everyone quietly agrees it's close enough.
The third gapNobody owns fidelity once the design file is handed off
Design owns the mockup. Development owns the build. The gap between them, the actual look and feel of the shipped thing, doesn't formally belong to anyone once the handoff happens. Small deviations creep in individually reasonable, collectively the reason the site never quite matches the deck, and there's no one whose job it was to notice.
This isn't an argument that developers don't care about craft, or that designers over-specify. It's that a static file plus a handoff meeting is genuinely not enough information to build from, and everyone downstream is doing their honest best with an incomplete brief.
Three things that make the gap smaller, not the headcount bigger
One person owns fidelity end to end. Not design, not development, someone with authority over both who's accountable for what actually ships matching what was approved.
Responsive states get designed, not assumed. At minimum the widths that matter get looked at deliberately before build starts, rather than improvised once real content breaks the placeholder layout.
Spacing and motion are treated as spec, not polish. If it's not written down as a number or a rule, it's not a decision, it's a guess someone downstream will make for you, and it'll be a different guess than the one you had in your head.
A deck shows what a page could look like. Someone still has to decide what it actually does, at every width, in every state. That decision either gets made on purpose or it gets made by accident.
The live page rarely differs from the deck because someone did a bad job. It differs because a hundred small decisions the deck never made got made anyway, by whoever happened to be building it that week.
Keep reading: Landing pages from Zenlio Studio