I asked my postal clerk about it and he also told me it must be an error in the rate quoting tool.
And the CP representative (not clerk) whom I phoned also acknowledged it is an error.
To my surprise, recped is right though, that when the choices come back for a given calculation, the "delivery confirmation" will appear under Xpresspost, but NOT under small packet or surface parcel for example - which I never noticed before.
However, in the same "options" section, if you request coverage for say $200, the small packet option does not appear at all and the reason is written below in red type. But if the calculator worked consistently, the small packet option would still appear and it would be written "coverage included for $100 only" or some such thing.
i.e., the calculator forces you to request only $100 coverage if you want to use small packet - so it should force you to NOT choose "delivery confirmation" if you want small packet to appear.