Why does the Gwened dialect differ from other Breton dialects?

The Gwened dialect (also known as Vannes) is distinct from the other three primary Breton dialects—Kerne, Leon, and Treger—primarily due to its unique stress accent and vowel aperture.1

While the other dialects align with Welsh and Cornish by placing the stress accent on the penultimate (next-to-last) syllable, the Gwened dialect features the stress accent on the final syllable. Additionally, the dialect possesses several other distinctive features, at least some of which are attributed to its close historical contact with the French language.2

These differences are significant enough that, during efforts to standardize the written language, it was often necessary to adopt separate norms for the KLT group (Kerne, Leon, and Treger) and the Gwened dialect.2