i OS 4 was particularly notorious for heaping too much on the plate of previous devices, resulting in many users complaining that they were better off not upgrading.
Since then, Apple appears to have worked hard to keep older devices from being negatively impacted by new software updates, paring down backwardly supported features when necessary.
And the mix of models from a single hardware maker, such as Samsung, LG or HTC, often changes several times within a year.
All these hardware designs are also typically designed independently from the software platform.
A year ago, Nokia experienced similar coordination problems with Microsoft after attempting to launch a simplified version of its N9 configured to run WP7 instead of Meego Linux.