Those are all good questions.
I think you might find it helpful to draw a distinction between hydration as a pure number and the actual capacity of flour to absorb water. As you know, the hydration of a dough can be stated as a number that represents the ratio of the weight of water in a recipe to the weight of flour in the recipe. Having such a number makes it easier to discuss dough recipes and the hydration aspects of such recipes. Also, since that number is a baker's percent number, it can be used with baker's percents for other ingredients to scale recipes up or down and to better analyze recipes. Without baker's percents, including the one for hydration, the dough calculating tools at http://www.pizzamaking.com/dough_tools.html
would not exist.
Millers and flour marketers also use hydration numbers in relation to the flours that they mill and sell. The main numbers that they use are "rated absorption values" and "operational absorption values". You can read about these numbers and what they mean and how they are used at the thread at http://www.pizzamaking.com/forum/index.php/topic,4646.msg39204.html#msg39204
and at Reply 9 at http://www.pizzamaking.com/forum/index.php/topic,8419.msg72940/topicseen.html#msg72940.
Without having some hydration numbers and recommendations from millers and flour marketers for their flours, bakers would not know what amounts of water they should start with when using different flours. In my case, when in doubt about what hydration value to use when using a given flour, I usually start with the rated absorption value and adjust it as needed based on my experience and results using the flour.
But having numbers on hydration, as useful as they may be, doesn't tell us how much water a given flour can actually absorb. Some of the factors involved in the capacity of flour to absorb water are discussed in the article incorporated in the abovereferenced Reply 9, notably the effects of protein, starch, pentosans and enzymes. However, there are other factors that are implicated in the capacity of flour to absorb water. These additional factors include the type and make of mixer used to make dough (e.g., stand mixer, food processor, or bread maker), the types of attachments used with the dough mixers (such as whisk, flat beater and C-hook or spiral hook), the mix speeds and durations, whether the flour is sifted before using, whether a classic Calvel autolyse or other rest periods are used, how the ingredients are sequenced into the dough making process, and environmental factors such as the moisture content of the flour, elevation/altitude, the age of the flour and how it was stored and, in some cases, the humidity that the flour adsorbs or desorbs at the place where the dough is made. The amount of dough that is made can also be a factor in the sense that if a dough batch size is too large or too small for the mixer used, the capacity of the flour to absorb water is compromised. Also, if a dough is made using hand kneading instead of a machine, the way the dough is kneaded and the duration of the knead will also be factors, as will any special kneading steps used, such as stretch and folds, punchdowns, etc. As you noted, using bench flour will also have an effect on the absorption of water by the dough.
Several of our members, notably Chau, have demonstrated an uncanny ability to incorporate a lot of water into just about any flour, far more than the rated absorption values of the flours in question. To show you that it is possible to hydrate a flour at a value close to 100%, see the dough recipe at http://sites.google.com/site/hollosyt/quickrusticciabattapizza
. I believe the lowest hydration value I have used was around 36%, for a cracker-style dough. As you can see from these examples, the hydration as a number does not tell us how much water a given flour can actually hold.