* are you sure the 30 mA are at 1.2V (as the 120mA in the chart)?
* you need to recognize: the lower the current, the higher the usable energy.
* the usable energy will differ from cell to cell
* the usable energy depends on cell type (chemistry)
* the usable energy depends on brand and exact type (model)
Some name-brand alkaline D cells have a little C cell inside with the C cell capacity. Few D cells are sold today.
Some Chinese alkaline D cells have anything that is cheap inside.