organic chemistry - How can weaker nucleophiles kick out stronger ones in the reaction of chloroalkanes with sodium iodide in acetone?

I have the following S_N2 reaction, in an acetone:
1,4-dichlorohexane (1 mol) reacting with NaI (1 mol) to form 4-chloro-1-iodohexane and NaCl.

However, chloride ions are stronger nucleophiles than iodide ions in an aprotic solvent and iodide ions are weaker bases than chloride ions. Reactions generally form weaker bases, why does this reaction occur? My text shows it does, but I don't understand why. What am I missing?