This is one of those cases where there are two hostas with the same name. One is registered and, therefore, the "official" version of H. 'Dumbo' while the other is a non-registered plant and needs to be renamed to avoid further confusion.
 

This cultivar of unidentified parentage was registered by Dennis Savory of Savory's Gardens in Minnesota in 2007. It forms a giant size hosta with wavy, blue-green foliage that has a slight amount of corrugation. Pale lavender flowers from late June into July.

Hosta Helper will call this plant 'Dumbo' (D. Savory)

A sport of H. 'Blue Mammoth', this hosta is a large size plant with blue-green foliage that has a golden yellow marginal variegation. It bears near white flowers from June into July. Michael Pinterics of Wisconsin in the originator.

Hosta Helper will call this plant 'Dumbo' (Penterics)