Eh, it's the trademarkeable thing going on.
Also, we don't really know how Tau adjectives work, so maybe "T'au" is the adjectival form of "T'au" and the difference is inferred contextually. Kinda like in English where articles can make adjectives into nouns. "a heavy", "greens", etc.
Hell, for all we know, Tau might be a tonal language, and that is what defines the difference. Norwegian sometimes does that, though only in spoken form, the written forms are otherwise distinct.