This is somewhat complex to explain. The magnetic field is based on two main factors... amount of current flowing, and distance from the wire. Removing coils from the outside of a large coil (where the field impact on the core is less than coils near the center) has less reduction on the magnetic pickup capability. Removing any wire will reduce the resistance, and increase the current... but not very much unless the wire is very fine. Wire has a pretty small amount of resistance.

So, when you remove the coils, did you decrease the resistance enough to increase the current enough to create more magnetic field across all of the coils given some reduction in field from the coils that you removed.

Make sense?

For the class exercise, you would probably get more increased field coupling by adding windings because the wire size is large enough that the added resistance is negligible.

Mac