Sounds like it might be reserving space for the top status bar in a different orientation.

Either this is a weird device specific bug or perhaps the author of the post launched Safari in a different orientation on one of the devices and then rotated (and Safari cached a different "screen" size).

This is complete speculation and I don't have a device next to me to test, but the 20px difference suggests that it's referring to the black status bar at the top.

Edit: looks like it might be related to tabs: http://stackoverflow.com/a/13380055/552710