The updates for both devices are developed by the OEMs, but there are no modifications to the UI and no carrier testing to get in the way. As a result, we get this super-fast update.
The update arrives merely a week after it was officially announced and started rolling out for the Nexus devices. In fact, many of the Nexus devices are still to receive the update, so you can say the Google Play editions are more or less on par with the Nexus devices in terms of receiving updates.
The HTC One kernel and framework source just dropped today, in fact. Hopefully all future updates come out this fast. The Nexus phones only started seeing Android 4.3 in the last week or so, and some users haven’t gotten the notification yet.
The Android 4.3 update was announced last week at an event, alongside the new Nexus 7 tablet and the Chromecast media streaming device. Although not a major update, it brings features such as restricted user profiles (only on tablets), Bluetooth Low Energy profile, virtual surround sound, autocomplete for dialpad, improved keyboard, OpenGL ES 3.0 support and a handful of other things.
Over and above these features, the Android 4.3 update enables Bluetooth tethering on the Galaxy S4 and the IR port on the One.