The concepts that you gained about growth limitation are based mainly on a carbonaceous nutrient such as glucose. The situation changes slightly for other factors such as nitrogen, sulfur, and phosphate. The problem arises because cell mass may not exactly reflect these limitations. For example, cells without adequate nitrogenous components will store carbonaceous ingredients for later use.
Some polymers form inclusion bodies within a cell.
Were we to plot cell nitrogen instead of cell mass, the graph would resemble that for cell mass in a carbon-limited culture.