they don't actually know the demand for all these new iphone Xx models. it's an educated guess. and... to some degree, a portion of the market will go wherever apple suggests they go anyway.

apple got caught 'behind' in the 'bigger phone size' competition, and it seems they'll keep going bigger.

you could keep the 'high margin' on an se-sized phone - people would buy it anyway, because they want that size.