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. 🙂 I worked for a company that never put out a .0 version of software. It was always something like 2.0.1. Makes it sound as though things have been fixed.
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!