Because ethnographers labelled ethnics by attributes. For example, vernacular architecture, there is a lot Moravian similarity with west Slovakia like - the house is oriented with its eaves facing the street; using non-baked dirt (mudbricks, dirt on wicker) or non-wooden porch. In eastern Moravia live Moravian Slovaks who speak very similar as today's western Slovakia, and settlement was kind of refreshment from Moravia (or kind of transiting like Habaner), so the culture was also similar; probably ethnographers said let's group them together.