So you imply one thing, the reader infers another. It can happen in small subtle ways, or great big clunking ones – the character you intend as a noble hero can seem more of a villain for example. Why is it hard for the writer to spot? Because it’s still you doing the rewrites, and you still have your initial perception colouring your view.
Here’s an example, not from art but from life, of this effect in motion.