I see different people use different hydrations for their dough. One person would use 57% hydration for AP and the other would use 63% for same AP. What is the reason for different hydrations?
It can take pages to answer this question but, to simplify matters, different styles of pizzas can call for different amounts of water for a given weight of flour. And even within a given type of flour, such as all-purpose flour, there can be a fairly wide swing of hydration values. Some flours can be used for many different pizza styles but the hydration (the amount of water relative to the amount of flour, by weight) has to be selected to be compatible with the pizza styles selected. That is how a flour such as an all-purpose flour can be used to make a floppy NY style pizza, a Chicago deep-dish or thin style pizza, a cracker style pizza, a Sicilian style pizza, a California style pizza or a thick pan pizza. Usually, each style of pizza has its "preferred" flour that is best to use for that style. For a further elaboration on this aspect, see Reply 6 at http://www.pizzamaking.com/forum/index.php?topic=9008.msg77976;topicseen#msg77976