Well the dock and the homescreen have very different behaviors, and for good reason too. While the dock automatically centers the icon layout no matter how many apps you place there, the home screen will always fit the icons to the predefined grid (4x5 or 5x4).
As much as I, of course, wouldn't mind options, as in, you could choose right on settings how many icons you want on your homescreen, if such a feature isn't in place, then thank god you can't fit six icons to a row just by throwing them there - it would be cramped as fuck instead of the nice, well spaced 4x5 layout I really appreciate, especially when I compare it to the iPhone.