That's a bit misleading. Spiders may grow more than one ring during a good year and less than one during a bad year. But overall, it should average out.

Seeing this is HN you might get downvoted for jokes.. Still, when reading the article and before getting to the part where they say the spider's entire life was monitored, I also envisioned a person in a labcoat cutting a leg and counting rings. Or performing some carbon-test like procedure.