The mappings are different because Mac OS X is not a DOS, for which the web site you linked to was apparently written. The specific mappings were inherited from previous Unix/Linux terminal emulators, as is reflected in your options for what kind of terminal emulator Terminal.app you wish to appear:
As you can see for example here, mapping
^[[17~ is widespread consensus among terminal emulators.
Shift-F12 is equivalent to the VT220's
Opt-F15 is equivalent to the VT220's
F20, i.e. option "shifts" 5 keys to the right.
I assume this mapping is the result of the desire both to be compatible with existing mappings, and to provide as close to a full 20-
F-key keyboard as possible with only 12 or 15 F-keys on modern Macs.
If you don't like these particular mappings, it's easy to change them in the settings dialog you already mentioned. Remember that you can map to pretty much arbitrary input (which is actually a pretty nic(h)e way to do Typinator/TextExpander-like keyboard shortcuts in Terminal):
If you prefer editing in a plain-text editor, you can export a Terminal setting via the cog icon, edit it (it's XML), and import it again.