To my understanding, strong consistency can be achieved when Vr + Vw > V. Vr is the read quorum (Vr), Vw is the write quorum. Assume V = 3.
When write a value (val = 2) to the DB, it only need to write success to 2 machines (e.g. Machine A, and B). when read a value from DB, it only needs to read from 2 machines if they return the same versioned value, in order to achieve strong consistency.
What if: after successfully persist val =2 to machine A an B, A went down and the value hasn't been replicated to machine C. Then when reading the value , it has to read from machine B and C, which has different value. Then which value will it choose as the latest result ?