[refurb]

Bingo!

These collection agencies purchased the loan for a fraction of their value (if paid according to the terms).

I'm guessing each collection agency has their own strategy for collecting on the loans. One of them says "we'll end up with more in the end if we don't charge interest" and the other said "we're owed interest, let's collect it".

I would imagine the first strategy (no interest) has the benefit that the person making payments can actually watch the debt decrease over time. It might make them more interested in actually paying the whole thing back.

If a collection agency says "you owe us $100K and next year it'll be $110K" you might just not bother paying anything since you'll never be able to get ahead of the interest.