How the two calculations differ
The first tool answers a question about a single conductor: how far its resistance rises at the operating frequency and how much copper sits idle. The model describes an isolated conductor, with no field from neighbouring turns, but the solution is exact.
The second computes a whole winding: it adds proximity-effect losses between turns and layers, which often exceed the conductor’s own skin effect. It uses Dowell’s model, accurate to ±20–30 % for a typical E-core window.
The usual order of work: first judge how poor a solid wire is at your frequency, then select a litz construction to replace it.