一篇简单的英语阅读John Smith and his friend Bill Jones were shipwreck

一篇简单的英语阅读
John Smith and his friend Bill Jones were shipwrecked on a deserted island in the middle of the Pacific Ocean.
They had plenty of fish and fruit to eat,but nothing to read except a book full of jokes.
Each of the jokes was numbered.
At first,to help pass the time,John and Bill read the jokes to one another ,then they told them from memory.After a year they knew the jokes so well that they just said the number of the joke.
“Twenty,” John would say,and Bill would roar with laughter.
Then Bill might say,” Seventy-five ,”and John would laugh.
One day,after they had been on the island for many years,another man was shipwrecked with them.
“How do you pass the time?” he asked them.
“We tell each other jokes.”Bill said,and handed him the joke book.
“Tell us a joke,” John said.”Just read out the number.”
“O.K.,” the man said.He looked through the book until he found a joke he thought was very funny.It was number eighty-three.
“I’ve found a good joke,” he said.”Are you ready?”
“Yes,” Bill and John said.
“Right,” the man said.”Here it is.Eighty-three.”
John and Bill just looked at the man.Their faces did not move.They did not make a sound.
“Why didn’t you laugh?” the man asked.
“You didn’t tell it very well,” Bill said.
这篇文章的表层意思我都看懂了,但是不太理解.就是说那两人为啥不笑类?

明明只是说序号,也能发笑……有点夸张地说两人被困,是在无聊,用老掉牙的笑话自娱自乐……
笑话没讲好,所以不笑,对应这种情况下就是那个数字没念好……囧……