Bathroom remodels pack a disproportionate number of trades into a small square footage, which is exactly why they're easy to underprice if estimated as a single lump figure rather than trade-by-trade.
Demolition and rough-in
Demolition and rough-in (plumbing and electrical relocation, if fixtures are moving) often costs more per square foot in a bathroom than in any other room, precisely because of how much is packed into a small space — moving a toilet drain or a shower valve a few feet can mean opening walls and floor in ways a similarly sized bedroom remodel wouldn't require.
Waterproofing
Waterproofing is a bathroom-specific line item that doesn't appear in most other room remodels — shower pan membranes, wall waterproofing behind tile, and proper slope-to-drain in a tiled shower floor are all code-relevant details that take real labor time and specific material, not just a general contingency allowance.
Tile
Tile (walls, floor, shower surround) should be taken off using the same flooring/tile waste-factor principles as a flooring estimate, but bathroom tile work typically carries a higher waste factor due to more cuts around fixtures, niches, and curbs.
Fixtures
Fixtures (vanity, toilet, tub/shower unit, fittings) are counted individually with their own price points, and it's worth pricing labor for fixture installation separately from the fixture cost itself, since installation complexity (a freestanding tub vs. a standard alcove tub, for example) varies the labor hours significantly even when the material cost is similar.