The OS update process, as explained by HTC

Here's a new webpage provided by HTC to give owners an idea of all that's involved in keeping devices up to date.  It provides information for which stage of the process they're in for HTC One devices.  http://www.htc.com/us/go/htc-software-updates/

Here's a direct link to the diagram showing the process (from Google update release to OTA download)
http://www.extremetech.com/wp-content/uploads/2013/12/HTC-Anatomy-of-an-Android.jpg

The diagram reaffirms that the process is slowed down by the OEM skin integration (HTC Sense) and by carrier customization, because unlocked and Google Play Editions are already running Kit Kat, while carrier variants require additional tweaking and certification before the software update roll-out can begin.