All you need to know about the Windows Phone 8.1 'Preview for Developers' | Windows Central
Quoting from the above:
What’s the difference between this and the official update? There are technically two parts when it comes to official updates: - An OS update aka Windows Phone 8.1 build xxxx.xxxx
- A firmware update specific for your phone and carrier (if sold through one)
The OS update is universal; it works across all devices and, for all intents and purposes the Preview for Developers is the same OS update as the official release. This is not a beta version of the OS. The one difference is the Preview for Developers does not include new firmware. All the firmware does is enable some device-specific functions, most notably new camera improvements on Nokia devices or the new SensorCore APIs, which will come with the Lumia Cyan update. With the OS update though, you’ll still get 100% of the all the 8.1 features announced, you’ll just keep your current firmware. What happens when 8.1 officially rolls out for your phone and carrier? Basically, nothing. Nothing in the good sense. Your phone will get the same update, but unless the OS is different, the only thing that will be updated is your firmware. After the update, you’ll be on the same page as everyone else. Now there is a lot of confusion getting multiplied; because the firmware does not bring features to most Lumia's in the Denim round (and thus no advance in the Firmware revision number) - and compounded by a glitch where extras + info reads Denim or Cyan depending on the P4D App being toggled on or off.
It was always stated quite clearly that Firmware is not (and can not possibly be) a part of the Preview for Developers.
What are they pushing in the P4D updates? Changes to the core OS.
P4D is one OTA push for all devices at once. Firmware is tailored for each and every device.
For Firmware you need an OTA push on a device by device basis. Never mind what it does or does not bring you. How can you get around that logic?
Saying that a generic, one-size-fits-all update brings you specific-for-your-hardware updates makes no sense.