OK that makes a lot of sense. So when you put broccoli in boiling water, it soaks in some of the heat that would ordinarily have contributed to the heat of vaporization of the water, thereby making the broccoli potentially much hotter than 100 celsius and retarding the rate that the water boils. I guess that's why it is so common to turn the heat down once something begins to boil, cause then you know you've got a fairly consistent temperature with which to cook the broccoli/whatever.