A spectrum suggests varying degrees of a single condition, and would be compatible with both the hypothesis that all occurrences of this condition share a cause and the hypothesis of multiple causes. So to claim there are multiple causes is different from saying that cases fall on a spectrum. It tells you that multiple conditions can appear the same way, so they get diagnostically grouped together based on symptoms even though the underlying causes, and so the appropriate treatments, can be quite different.
A spectrum suggests varying degrees of a single condition, and would be compatible with both the hypothesis that all occurrences of this condition share a cause and the hypothesis of multiple causes. So to claim there are multiple causes is different from saying that cases fall on a spectrum. It tells you that multiple conditions can appear the same way, so they get diagnostically grouped together based on symptoms even though the underlying causes, and so the appropriate treatments, can be quite different.