Perhaps the builders were trying to maximize the villa's exposure to sunlight in those high latitudes. The eastern walls in both wings would be warmed by the morning sun while the western walls would get it in the afternoon.
It's a cool, damp climate, one in which the Romans might have craved sunlight, so unlike the Mediterranean where they would've sought shelter from it.
The Romans came up with the hypercaust system, and used it in particular in colder places (such as Britain), so if this lacks it, that might be even more unusual.