> It's just a bare minimum that you reduced
No, it's the -maximum- that I reduced entropy by. If I choose randomly from 4 possible passwords, I've shaved off --no entropy--.
If I choose with a strongly predictable preference (say, everyone knows I'll pick the password that's first alphabetically) I've shaved nearly 2 bits off the probable search space: you probably only need to look in the first 1/4th of the search space.
Choosing with some subtle, individual preferences is something inbetween.