I believe that there is a theoretical maximum on the renown (people have reported that over 2000 renown it gets increasingly difficult). I presume that the diminishing effect of this cap is not stated in the calculations before the tournaments/battles and will be when you actually get the renown.
This probably results in the effect you are having, you get 24 renown from the battle stated, before the diminishing returns are calculated.
The formula to calculate renown works as follows:
The higher your renown, the less renown you gain for the same activities. Any time you gain renown, the game takes your previous renown, divides it by 200, then subtracts the
result from the renown that you just gained.
Or mathmaticaly stated:
Renown from event = renown stated at start-(current renown/200)
This forum post gave me some information, including the formula above: http://forums.taleworlds.com/index.php/topic,115829.0.html
I do hope this answers your question.