What??? You ask. Hear me out.
First, numbers in most contexts are adjectives. Adjectives don’t show number in English. We say “five apples” but not “fives apples.” But that’s not my point.
Let’s move on to arithmetic. We (correctly) say “Three and six are nine.” Plural verb, so plural numbers, right? Not quite. That sentence has a plural subject, three and six. You could as easily say “Tom and Pete are sick.” The two persons are one each, and they make a plural subject.
A number is singular when you talk about the number itself!
For example, you say, “six is half of twelve, thirteen is a prime number.” Singular verbs! You’re referring to the number itself (not themselves), not six of something, such as six people.
Finally we get to the comic. Third panel. The guy confuses referring to the number itself with the number of things. Sounds wrong, doesn’t it?
Anyway, there’s a little incongruity for you that I bet you never noticed.