Specifically, I noted one place where I thought a comma should normally have been. Perhaps it's "paranoia", but it seemed like that is one possible "variation" that could have been used.
I don't know how many Apple employees would view this memo (it seems unlikely that all ~135,000 would) but it doesn't seem like it would take very many slight differences like this to be able to generate a unique version of this memo for each viewer.
At that point, Apple just has to sit back and wait for the memo to leak. Compare the version of the memo posted in TFA to the "unique versions" rendered to the employees and you've either identified the leaker or, at the very least, significantly narrowed down the possibilities.
If Bloomberg were being careful, they would attempt to obtain copies of the memo from multiple "leakers" and compare them very carefully before publishing, making sure to look for these minute differences between them. If any were found, they'd have to be very diligent when posting the memo for all the world to see -- if they were being careful and if they cared about protecting the leaker's identity (one would assume they do but I think it'd be safe to assume there's a limit to how far they're willing to go).
Regardless, it's pretty clear that this is a huge attempt by Apple to deter any leakers or potential leakers from doing so.