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IDLE contains keyboard configuration in config-keys.def (Mac, Windows, UNIX) and in configHandler.py. 
GetCoreKeys contains the "keyBindings" dict which has fall-back values in case the given key set is missing values (a warning is printed). Ankit's screenshot contains these warning messages.
GetKeyBinding is a helper function that splits a string containing multiple key bindings. Read the commented header in config-keys.def to see why this is so. 
The fall-back values in keyBindings do not pass through the GetKeyBinding function, and thus "<Key-Return> <Key-KP_enter>" does not get split.
Everyone else uses IDLE's prepackaged key bindings as a starting point for creating custom key maps. The key maps from config-keys.def already passed through GetKeyBinding. This is why <Return> works for everyone else.