What delivery actually involves

The home is transported in one or more sections on a specialised trailer. Depending on the route and the size, that can require permits, escort vehicles, and specific times of day. This is arranged by the dealer or a transport company, not by you, but the timeline is yours to live with.

Multi-section homes arrive as separate pieces on separate trips and are joined on site, which is a different day's work from the delivery itself.

Where timelines really go

Build slot. If the home is ordered rather than on the lot, the factory queue is usually the longest single stretch.
Site readiness. Foundation, utilities and access have to be finished before anything is delivered. This is the most common reason a delivery date moves.
Permits. Transport permits and local placement permits are separate things on separate clocks.
Weather. Not an excuse, just a fact. Soft ground stops a transport truck.

Questions worth asking early

Who is responsible if the site is not ready on the delivery date?
Is delivery included in the quoted price, and to what distance?
Has anyone physically looked at the access route?
What happens if the home is damaged in transport, and who handles the claim?