Because, I think that a few kites might have the "Y" suited for good performance, others are not sensitive.
And others might just change behavior a bit - but not necessarily to the worse
I would personally just go with the 4 lines without the Y and see how it works - as I honestly think it will work really good on almost all kites in fact
Some might say that some kites are designed to work fine only with the delivered "Y" distance.
But it doesnt have to be true for all of them
You dont write what size - as the bigger the kite the more influence you get when changing the "Y" point (more or less pinching)
I would not worry with your kite, and just do it