I've yet to withdraw from a seige but in my 1st Parthian campaign I was under siege by Egypt. My army inflicted heavy casualties on thier main assualt force. They had another, smaller force waiting out of arrow range that withdrew when they saw the death of that force, I was granted a victory but thier 3-4 units that withdrew where untouched and still existed on the camp map...so is it only working for the AI or it's something to do with army size?