No, kids are not 'supposed' to come in, knowing everything.
Not all kids, go to Preschool, not all kids are at the same rate, of attained 'academics.'
It is also that each school has a different curriculum.
My son is in Kindergarten now.
They do: the calendar, the weeks, the months, ABC's, sight words, reading & phonics, writing, coloring, they learn Japanese and Hawaiian language, they have music, they even have computer class, art class, P.E., they have reading groups, they do number related things too. And they have 'jobs' in class, as class room monitors for various things.
And my son gets homework. And they have a nap time as well.
My son's teacher said, that each child is at different rates of ready knowledge. BUT, from her experience, the Kinder kids, seem to all get in the groove... by about Winter break time.
Some kids come in knowing how to write, and some don't. Some come in knowing how to read in various levels, some don't. But they have a curriculum, that they teach. And a Teacher will know, each rate of ability, for each child.
My son, did NOT know how to write... upon entering Kinder. He had NO interest in it. Even though he went to Preschool. His Teacher told me "DO NOT WORRY!" She isn't. But now, my son IS writing. That is what Kindergarten is for.