Two open-ended thin shells of the same axial length \(L\) are centered at the origin along the \(z\)-axis: a circular cylinder of constant radius \(R_0\) and a truncated cone whose radius varies linearly from \(R_0 - \Delta R\) at \(z = -L/2\) to \(R_0 + \Delta R\) at \(z = +L/2\), where \(\Delta R < R_0\) and \(L \ll R_0\).

Each shell carries a steady, purely azimuthal surface current with identical uniform current per axial length \(K\).

A magnetic sensor placed at the origin measures a field that is directed purely along the \(z\)-axis and has a greater magnitude for the cone than for the cylinder.

Which of the following provides the correct physical explanation for these observations?