You make your point, but I just don't see Apple having a hard requirement to ship 4.2 prior to 4.2.1.
One could almost consider 4.2 to be 4.2.0. Oh no! A .0 release! No one wants that! So put out a 4.2.1, and it's no longer a .0 release.
But seriously, there's no reason they have to release 4.2 when 4.2.1 is about to be released.
There's nothing magical about a continuous sequence!