Al-maʿnā al-wāḥid refers to the particular meaning a speaker intends to convey. It does not require the alternative statements to have identical incidental details.
If the intended meaning is generosity, compare:
“Zayd is generous” — the quality is explicitly stated.
“Zayd keeps the hearth lit for his guests” — regular hospitality suggests generosity indirectly.
The second expression also evokes a hearth and guests, details absent from the first. Those differences do not prevent the expressions from sharing the same principal communicative purpose. Unity concerns the intended target, not every secondary association.