On the iPad, when you change orientation from landscape to portrait mode, only 8 of your app icons out of 20 stay in relatively the same place -- the 4 top, leftmost icons and the 4 bottom, rightmost icons. That means 12 of the icons -- more than half -- change positions, and 5 of them change rows completely.
iPhone has never done landscape mode home screens, but even if it did its 4x4 icon grid would stay the same no matter which orientation it was in. iPad's grid, however, is 4x5 in portrait and 5x4 in landscape. It's a rectangle not a square.