That's one of the interesting things about language. It's all just sounds that we agree represent ideas.

But, because language use is such an intrinsic part of our brains, the rules around language are picked up much faster than we realize. This test is generally done with very young children that haven't had much (if any) grammat traint. They just pick you that adding s to the end of a noun means that there are multiple of if. They'll use that rule even when the noun is nonsense.

But that rule is arbitrary to an extent. We could collectively agree that adding k at the end means plural. It doesn't even have to be at the end, it could be anywhere in the word.

Some words don't follow the normal rules. Like mouse, and mice when talking about the animals. Or the exceptions I mentioned earlier.

If we apply those exceptions to a nonsense noun like wug, it is no more or less "right" than adding s. But the test is about showing how language develops, not how a given language functions.

English is an odd language sometimes though. We borrow words from other languages, sometimes adopting the grammar and rules, sometimes not. But English is built on multiple older languages to begin with, so the rules it has can be mind boggling.