It's the remnants of the BSc being a significantly less popular course back from when it was still taught at Stockton. Now it's being taught at the Durham City campus, the numbers have evened out a lot (there's less than 30 people in my year - going into third - from the BSc and Health and Human Sciences combined, though there's a more even split, though still a little more popularity in the BA, in the year below now they've combined the department into one campus). As you say, there's little difference between the two - I think most of the difference comes in second year, but even then they're minimal. I also think it's relatively easy to transfer between programmes, though the department's administration can be difficult to deal with at times (I'm currently trying to sort out some issues with my own third year modules and finding them rather unhelpful, but I've had very easy dealings with them as well).