Pagoda Company manufactures metal picture-framing materials. Part of the manufacturing Obj. 1 process is the coating and painting of the metal with finishes. Management of Pagoda believes the painting process is a variable cost that is incurred evenly for each unit that passes through the painting operation. An analysis of several recent production runs revealed, however, that costs differ on a per-unit basis when the run sizes differ. Is the cost of painting a variable cost? What explanation could you provide for the cost behavior of the painting operation?