Yesterday came up with this theory: Generally, people can be divided into those who're actively searching for humour; and those who only see humour when it's easily recognised as humour, perhaps because it is presented by a comedian (or someone with a reputation for being funny) or because everyone else is laughing.
How did I come up with this? Hugh was saying how he used to not smile or anything when he was making a joke, and so most people just wouldn't realise there even was a joke. Now he does smile when telling jokes, and so more people can appreciate his humour. I remember I used to do the same thing when making jokes.
Also, because Hugh just seems like such a serious person, I'd forgotten what a funny person he actually is until yesterday.