This table is based on KBART reports from large, independent, long-term digital preservation projects. We use the start and stop years of fulltext coverage, then count individuals works as included or not on the basis of year alone (not considering volume or issue metadata). These are mostly "dark" archives, with no direct public access to holdings.
The Keeper's Registry project, currently run by issn.org, is a more authoritative source for aggregated KBART reports, and includes more archives.
You can double check the Keeper's entry for this ISSN on portal.issn.org; click through to the "Archival Status" link to see holdings information.