Using the top 100 lists at the main three prog ratings sites, it looks to me like the most egregious omissions are mostly albums without English lyrics.
Anglagard's Hybris is #17 at Progarchives, #70 at RYM (for "Progressive Rock"), and #10 at Gnosis (or #5 if you remove the re-releases).
PFM's Per Un Amico is at #16 at Progarchives, #55 at RYM, and #6/#3 at Gnosis. The first PFM album isn't far behind at all three sites. A few other Italian albums (Banco, Museo Rosenbach, Le Orme, etc.) are in the top 100 at all three sites too.
Maybe there was some rule excluding albums with non-English lyrics?
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