According to the AOSP source code, the big character shown at the first stage of the KitKat easter egg screen is the first character of the build ID. You can find the build ID (called “Build number”, but actually it is not just a number) on the Settings → About phone screen.
On Google Nexus devices the build ID for stock KitKat ROMs looks like
KTU84Q, therefore the big letter on the easter egg screen will be “K”. In fact, the first letter of the Google build ID always matches the Android version code name, and the code structure is documented.
Other manufacturers have their own build ID formats. E.g., for the Sony Xperia E1 model the build ID for one of KitKat ROM versions is 20.1.A.2.13, and for the Sony Xperia M2 Dual the build ID may be 18.3.1.B.0.18; this explains why the first model shows a big “2” on the screen, while the second shows “1”.