Does anyone understand how the Muamar vehicle contract rewards actually work?

I'm looking for certain rewards from the end of the Just Another Story gigs. But it seems no matter what cars I take and how many times I attempt them, certain garage containers always have the same TYPE of rewards.

For example, i'll take a car either from Downtown, Watson or Japantown and if the destination is the garage locates in the Badlands, the rewards will always be a pistol mod, a SOR-22 and a Yukimura. I'm thinking the reward information is tied to your save game, because on other saves my V would get different rewards from the chests (but similarly they were the same per-location.)

If anyone knowledgable about how this system works could lend their knowledge, that would be excellent!