3/8" (roughly 9.5 mm) is indeed impressive x-max for a 5 1/2" midwoofer. It is harder to build that much excursion into a small-diameter suspension system than into a large-diameter one. In general there's a tradeoff relationship between long excursion and good top-end extension, but sometimes a dedicated designer can find a way to meet seemingly conflicting goals.
Duke
dealer/manufacturer/no affiliation with Totem
Duke
dealer/manufacturer/no affiliation with Totem