A reading, once published, is never rewritten. If a number has been cited, it has to keep meaning what it meant, so each period is frozen at its own URL with the value and timestamp it had at publication. Corrections to an open period go to the current page; the recorded page for that period stays as it was.

This is why the archive is worth having as a list rather than as a scattering of links: it is the evidence that the figures were stable, not merely that they exist now.