Source: Morrison and Boyd - Chapter Alkanes - exercise no. 15

Free-radical chlorination of either n-propyl or isopropyl bromide gives 1-bromo-2-chloropropane, and free-radical chlorination of either isobutyl or tert-butyl bromide gives 1-bromo-2-chloro-2-methylpropane. What appears to be happening? Is there any pattern?

I have really no idea how this can be happening.