Ziyadat al-thiqah is not restricted to adding a word to the matn (the report's text). It may also involve an additional transmitter or connection in the isnad (chain). In both cases the question is which version is accurately preserved.
Textual addition: One version adds a qualification or descriptive phrase to a ruling. Examine whether it coherently fits the other versions.
Chain addition: One version names a Companion in a link where another appears to omit that connection. This may belong to an inquiry about a connected versus a mursal chain.
Even here one cannot assume that a longer chain is necessarily the more accurate chain. Comparative evidence is needed.