It's a known issue on byzantine music: the hard chromatic often becames soft, when the phrase is not defined by the tetrachord pa-di (triphony), but by the trichord pa-ga (diphony). In that case the pa-ga is chanted more softly, like the soft chromatic, you are right. Sometimes, in newer book editions, this is indicated. But generally it is not indicated and it's performed like this by tradition. And this is natural: the voice tends to perform soft intervals on diphony (because the top note of pa-ga is a diphony from the base and the melodic attraction is based on that, so Vou is higher, as you say) and hard intervals on triphony (because the top note of pa-di is a triphony from the base, so Vou is lower, the one of the hard chromatic).
A very well known co-existance of the two chromatic genres is performed continuously on heirmological plagal second mode. In this mode you start with diphonic phrases and soft chromatic genre and you finish with triphonic phrases and hard chromatic. These details are not described thorougly on theory, you have a reason when asking. Maybe on some theoretical books are referred, but I doubt if there is a book that describes all these details. Your guide is -and always will be- the oral tradition. And not any tradition, but only well known traditional shanters, mostly old ones, like Nafpliotis, Priggos, Stanitsas, Chrysanthos Theodosopoulos, Tsamkiranis, Firfiris and many others.
