An apparently unheard-of hilarious family joke

I was talking to my grandad just now and he brought up a joke that all my family has heard a million times before, but apparently no-one else has heard of.

The new prisoner that was brought in today overheard his inmates doing something very weird.
"forty!" one called out. everyone rolled around laughing.
"thirty-five!" some chuckled, others rolled their eyes.
"fifty-two!" that one seemed to be hilarious.
The new prisoner nudged the one he was sitting next to: he didn't seem as menacing as some. "What are they laughing at?" he asked.
"Oh. That. Well some of us have been in here for a jolly lang time, and we have heard some jokes so many times that we just gave them numbers to save ourselves explaining them"
The new prisoner decided to try his luck.
"fifteen!" he yelled. No-one laughed.
"Was there something wrong with that one?" he asked anxiously.
"No, no, it's just that we've all heard that one before." his friend assured him.

There was also some other jokes that my grandfather always brings up:

When is a door not a door?
When it's ajar.

Why did Six not like Seven
Because Seven ate Nine. (seven eight nine)

What did the big firecracker say to the little firecracker?
"My pop is bigger than your pop!"

What did the pig say when the butcher grabbed him by the tail?
"That is the end of me!"

And that is the end of my post! Hope you found it hilarious, and if you didn't, sorry for wasting your time!