iOS 5 for iPhone secretly contains iPad calendar features? The iPad calendar is much better than the iPhone version.
Arguably, with the Retina Display the iPhone should be able to display a similar interface.

All of the current iOS devices have different feature sets, yet claim to support the same operating system: iOS 5.

As of this writing iOS 5 is available for all iPad models, iPhone 3GS, iPhone 4 GSM, iPhone 4 CDMA, the iPod touch 3rd generation and the iPod touch 4th generation.

When the iPad was introduced, it came with iOS 3.2, which was a version made specifically for the iPad.  It included several features that would later be incorporated into iOS 4 and made available on other devices.

When iOS 4.2 (I think) was released, it marked the first version of iOS that was compatible with both iPhone, iPod Touch and iPad.

My question is this:

Does each device have a build of iOS created specifically for their hardware with every update release?
Or does the firmware file for the OS contain all of the features/interface elements for the entire product line (Like the iPad calendar interface) and only make available what features/settings the device should have?

Note, I know that iOS 5 can pull small updates and doesn't have to download the entire firmware each time. I'm asking about an initial upgrades and installs.
iOS 5 comes in different "flavors" for each device.  iOS is stored on the computer you sync to as an .ipsw file.  These files are located here:

~/Library/iTunes/iPod Software Updates/

~/Library/iTunes/iPad Software Updates/

~/Library/iTunes/iPhone Software Updates/

Each device's "flavor" of iOS is optimized and only contains the code needed for the device it pertains to.