Castro-Urdiales is not in the Basque Country, and Euskaltel does not operate there (with fixed connections). However, it is a suburb/population center for those working in the neighboring Bilbao etc. Barakaldo is also a major population center in that area.
That makes me think this is some kind of consumer-purchased device that is advertised/popular primarily in the Basque region, but not tied to ISPs directly. People would buy it and take it home. This would explain the disproportionate counts from Euskaltel (biggest ISP in the region only) and Castro-Urdiales (not in the region, but lots of people commute from there).
(I used to live in Castro-Urdiales)