site:edaboard.com al cupper
I think your 2. item is correct: But not the "difference" of the "reduction factor" (which includes under-etching, optical resolution errors and other process-dependent deviations), but its constant contribution, which by this creates the width-dependency.
For very small structure sizes, also the width-dependent relative (negatively counting) part of the non-conducting oxide skin (Al2O3 on Al) may contribute to this effect (width-dependency due to the relatively different "contribution" of the oxidized side walls).