Imagine the flavor of Grape Kool-Aid. Now think about grape-flavored candies, gums, etc. There's all one consistent flavor through all of these; it may vary slightly, as much as, say, cherry flavor varies through the same. But it's still one consistent flavor.
Now think of the flavor of grapes. The actual fruit. There are many varieties of grapes with many different flavors... and none of them has any similarity to the flavor discussed in the previous paragraph.
Sure, the Kool-Aid and candy flavors are never the same, not even close, but you can at least pick up some semblance of similarity. Apple doesn't taste like an apple, or like apple juice, but you can see how it could make you think of apples. Banana is usually more like sweet and slightly unctuous yellow, but there's a memory of banana in there. But grape seems to be emulating some specific mythical fruit that is no more like grapes than like any other fruit you could name.
What's weird here is not that it bears no resemblance to real grapes, so much as the fact that all these different "grape-flavored" flavors bear so much resemblance to one another. Somewhere in a lab one day someone made a flavoring that they decided to call grape, despite it having no similarity to any actual grape product, and for decades ever since, all grape-flavored products have followed that single vector.
I recently noticed that there's also a faux-vanilla flavor that multiple disparate products use that is entirely unlike the vanilla of actual vanilla beans and their extracts, or even like the vanilla of vanilla ice cream. It's not nearly as universal, though. I noticed it in a "vanilla-flavored" chewable lactase pill I take for my lactose intolerance, and while thinking of how it bore no resemblance to vanilla, it occurred to me it was similar to the vanilla flavoring in other chewable medicines and supplements; but more oddly, it's similar to the vanilla flavor of Ovaltine and malted milk powder.