That sounds like a good question. I don't really understand neither the APR or the conversion premium, but I guess I'll have to look the terms up new in the context of steem.
Btw, what happens if different witnesses have different settings for somethign like that?
There is generally a consensus mechanism which decides on the actual value. I believe in most cases it takes the median of witness values, but the premium works a little differently because it was done in an ad hoc fashion to avoid a hard fork.
Thanks for the clarification. I'm still going to read about it a bit more, but this makes it a bit clearer.