Because the seats are staggered!! I can see this in the photos.
As regards seat plans, it can clearly be seen that 2 business seats take up roughly the same width as one first class seat in the cabin in front. This is as clear as day. Print the seat plan and draw some lines with a ruler if you must. The staggering and row numbering policy seems to be confusing some posters.
Also, the seat plan in the link above clearly states that the Emirates A380 business class seat is only half an inch wider than the economy seat. Business class seats on Emirates 777s are 2 inches wider, but there are only 7 across.
Back on topic, BA business on the A380 top deck is 7 across in a 2-3-2 staggered layout, but not all forward facing or direct aisle access. On the main deck, the same seats are in an 8 across 2-4-2 layout because the cabin is wider.
For the record I have not flown in either cabin, obviously, and I am basing my comments on seat plans, photos and articles from respected publications discussing the subject.