The area on a deed is not the area you can build on. Developers and planners step it down: start with the whole lot, take away the land the law or a recorded right keeps free, then take away what the project itself needs for streets and stormwater. What remains is where buildings can go.
Wetlands are often the largest cut. Filling wetlands that are waters of the United States needs a permit from the U.S. Army Corps of Engineers under Section 404 of the Clean Water Act, and states and local codes often add their own rules and buffers, so plans usually keep off them. On the coast, tidal land can carry a separate state jurisdiction; in South Carolina, for example, the state regulates a critical area along tidal waters. Some codes also leave wetlands out of the acreage used to count allowed density, so they reduce both where and how much you can build.
Easements are recorded rights of others over part of the land: a utility line, a drainage way, a shared access drive, a conservation easement. A building generally cannot sit on an easement, and a strip across the middle of a lot can split what is left into pieces too small to use.
Then come the rules of the zoning code and the needs of the project: setbacks from lot lines and from wetlands, buffers, protected tree zones, and the land a subdivision gives to roads and stormwater ponds, which can be a large share of the site. The result is often a fraction of the starting acreage, which is why the realistic yield of a site can be far below its paper yield.