9.3.4—That the resistance of a filament lamp increases

Increasing current heats a filament, raising its temperature and increasing resistance because lattice vibrations scatter charge carriers more strongly.

Read the curved I–V characteristic rather than assuming V/I is fixed. Cooling on reduction can make the path history relevant.

At higher voltage a lamp’s current rises less than proportionally, showing a larger operating-point resistance.

The lamp is not non-ohmic because current stops; its resistance changes as temperature changes.