That's because not every single line is perfect iambic pentameter. Line after line of perfect meter ends up sounding more like a nursery rhyme than poetry, so poets (and playwrights) will play with the meter to emphasize certain words. Let's skip back to one of the non-perfect lines in the Prologue:
Hull is a town in Plymouth County, Massachusetts, United States, located on a peninsula at the southern edge of Boston Harbor. Its population was 10,293 at the 2010 census. Hull is the smallest town by land area in Plymouth County and the fourth smallest in the state.
Hull(verb) the outer covering of anything, particularly of a nut or of grain; the outer skin of a kernel; the husk. Hull(verb) the frame or body of a vessel, exclusive of her masts, yards, sails, and rigging. Hull(verb) to strip off or separate the hull or hulls of; to free from integument; as, to hull corn. Hull(verb) to pierce the hull of, as a ship, with a cannon ball. Hull(verb) to toss or drive on the water, like the hull of a ship without sails.