That's the impression I got: you were coming in from outside the area to deliver something to Primm, the unknown courier saw your name on the list of others who were due to arrive soon and since he flaked out (for reasons we still do not know) you ended up being offered the job when you arrived. You being new to the area would also explain why you took the north road through Goodsprings, the people in Primm might not have known about the Deathclaws and didn't warn you.
However I still wonder why the dead courier with work order 4 was back there in Primm after making his delivery, nor do we get to meet any of the other 4 couriers. If this info was all being saved for a DLC it'd be nice not to have to wait months to find out.
I'm fairly sure the guy who passed on the job knew who you were, and may even have known what the chip was; the storekeeper in Primm said the guy was shocked/freaked to see your name on the list, and insisted the chip be passed to you. I agree the fact that he is never mentioned or heard about again is kinda bizarre, and hopefully something they address eventually.
As for the other courier being back in Primm, I figured he had just dropped off the decoy he was transporting and was looking for more work when the convicts showed up and sacked the town. As for the other four guys, who do you think carries your stuff when you drop it in one of the shipping boxes?