I may be wrong, but I think that nearly all fruit trees you buy these days are grafted. They can get a lot more out the door by having lots of tree rootstock available, then when they get a new variety, they can graft to create them much faster than if they had to grow them from seed.

Its rare to even find one that isn't grafted. The rootstocks are chosen for disease resistance, and in some cases, the size of the mature tree, with 'Standard", 'Semi-Dwarf', and 'Dwarf'. Although I don't think I've ever seen a "Standard" peach.

I just purchased two standard peach trees, both grafted. One is supposed to be hardy to zone 5, the other to zone 4. I live in zone 5A, reportedly. Since I just put the bare root trees in the ground a couple of weeks ago, I don't know how they'll do come winter, but they are guaranteed for a year by the place I bought them from. I believe grafting is also how they guarantee the type/quality of fruit. For example, you can't just take a seed or a pit from a good piece of fruit and plant it expecting the same type/quality of fruit. If an apple was pollinated with a crabapple, you'd get fruit, but when you plant the seed, it would be some type of cross depending on the pollinator. With the grafts, they are taking a branch from an existing tree and putting it on the root stock of another to create an independent tree that is genetically identical. The grafting and rootstock also determine size, as has been said, and cold tolerance. One of the factors of cold tolerance is achieved through blossoming later in the year to avoid late frosts killing off the developing fruit. I'm probably speaking way above my expertise, but things I've picked up from various sites before I bought my trees.