Get out a tape measure, it will all be clearer.
As @BassBus says the 12 fret is fixed. For a 34" bass it's 17 inches from the nut. Typically the saddle for the E string will be somewhere around 34 1/2" from the nut.
(The first harmonic's node will be at 17 1/4" rather than 17", but we don't notice that.)
When fretted at the 12th fret the free length is 17 1/2" . That extra 1/4" of free length lowers the pitch of the string by enough to compensate for the increased tension when fretting.
To prove the effect, just fret the E-string at the 12th fret, pluck it and bend slightly, by the same as the action, say about 1/8" to 3/16" (or 1/4" if you are Carol King!)
The change in pitch you here is pretty much the amount the saddle displacement has to compensate for. It will probably be rather more than half a semitone on the E string.
Bend the G string by an amount equal to its action. the pitch change will be a lot smaller, which is why the saddle movement is less.