Alignment is usually asserted. This is one of the few places it can be measured: every state casts a recorded position on the same questions, in public, every session.
The cell is the share of votes on which both states recorded the same position. Abstentions count as a position and two abstentions agree — a defensible choice, and one that changes the answer, which is why the full method is set out on the methodology page and the underlying vote records are published.
High affinity is not friendship. Much of the General Assembly's agenda is consensual, so nearly every pair of states agrees most of the time; the interesting structure is in the differences between cells, not in their absolute level.