Arena doesn't clear out dead heroes (maybe all enemies, not just heroes)
If you drop some captured heroes to arena, they will fight to the death. However, when they die, the total count of creatures at arena doesn't reduce.
As you can see, there are 4 fighters at arena, but the room thinks it has 11, because there were 11 in the beginning. The only way to reset the count is to sell and rebuild arena completely.