Conceptual question about activity level and molar concentration?

activity is activity, not activity level. In an ideal system, activity is equal to concentration (mole fraction), but if there are extra non-ideal interactions, it is most typically the case that activity is less than concentration. A substance that forms ion complexes will have a chemical activity for the main ion that is less than the concentration of the element (some of the element will exist as complexes and not the naked ion). Even if you account for the existence of those complexes (and deal uniquely with activity and concentration of Al+3) the deviation is usually to a lower activity than ideality (concentration) would predict. The cation interacts with the solvent (the water, and perhaps other ions). It is possible that activity can be increased above what would be predicted from ideal behavior. That does happen sometimes.

Due to the electrostatic attractions between Al³⁺ and NO₃⁻ in the ion pairs, the activity of Al³⁺ ions would be lower than the molar concentration.