What does it mean to have an assumption built into your form? During evolution, organisms always existed in a certain place and time, with a given environment. As organisms evolved (in that place and time) in response to changes in their environment, certain aspects of the environment became the assumption behind the new form.
For example, frogs evolved to detect fuzzy dots in the sky and to leap toward them with their mouths open. The expectation (built into the frog's form) is that fuzzy dots are flies for a meal. In this sense, expectation is another way of saying "circumstances from long ago that are assumed to have remained constant" and thus are implicit in the frog's design. (Also, note that "design" only becomes apparent in a specific environment -- if fuzzy dots are not flies, the frog will soon die).
Present-day decision-making also involves expectation. A general may command his army to dig trenches or prepare artillery positions and wait for his next command. Once these are in place, the general can expect that his simple order canl be detected by the prepared troops (verbally, or electronically, or visually), and once received, that signal will trigger a great battle.
Compare this to hormone receptors, which (like a general preparing for battle) are strategically placed throughout the body to await a signal (a few drops of hormone), for which they already know how to respond. Once that hormone is released into the bloodstream (like a general's order), those receptors detect the signal and enact their scripted response.