The HTC US site has just posted an interesting update which explains a little of the process of what happens with the update process from the viewpoint of a major manufacturer.
The infographic is fairly large – click on the image below to have it expand fully – but lists the step by step process from a US standpoint – but the concepts are still valid – from the release of the framework through the Partner Development Kit(PDK) through to the implementation of the HTC Sense framework and certification process.
It’s a fairly long read, and pretty much explains why the update process can be a fairly lengthy process, although Motorola has been blowing this up recently with timely and extremely fast updates for their Moto X in the US, where the Verizon version actually beat the Nexus 4 to receiving Android 4.4.