...why?

Well, in writing -- Internet discourse in particular -- multiple exclamation marks are occasionally (ab)used to indicate absurd levels of emotional intensity on the part of the speaker. The factorial operation is also denoted by an exclamation mark; to someone unfamiliar with that fact, or someone capable of intentionally conflating the two usages, it appears that the statement "9945 = 17!!!!" represents someone excitedly asserting a falsehood (i.e. "9945 = 17") or possibly suggesting that shouting a number somehow increases its value.

Each line on the linked article is a more-or-less stoic mathematical fact, and the humor comes from the subversion of this expectation when the entry for 9945 is misinterpreted as I described.