The lack of identicality of the fries themselves turns out to be a part of McDonald’s’ broader engineering, as a matter of fact. (You could have expected this—this is the same company whose chicken nuggets come in well-defined named shapes—but it’s not terribly well talked about.) If all the fries at McDonald’s were uniformly long they would have bad stacking properties and would give you too much in your fry cup when filled, etc., so any new potato brand that they accept, or any change to their procedures for cooking their fries, must pass a bunch of stringent tests including having a certain distribution between shorter and longer fries.
In this case there is a page on McDonalds.com which specifies the only four monocultures which they exclusively farm to make McDonalds fries.
I might be wrong, but this is how many edible crops are propagated. Apples for instance are generally clones, because the alternative is getting a new kind of apple every time.
I'd love to see any info you have on that, because I'm not seeing anything at all. My Google Fu may be weak, but all I find is that McDonalds continues to deny using GMO potatoes.
Also, anecdotally, McDonalds french fries are far from all identical.