I suppose it is like it says 'reversal' in that a payment or some such as that has to be reversed, subtracted.
My guess is, this could possibly happen if say an advertiser purchases advertising and then the payment was
fraud or did not go thru completely. I don't think I have ever seen a reversal in my affiliate stats page in all
the years I've been a member. I'd say it is just there in case it does happen, you would be able to see the
details of the reversal.