Sometimes to reduce airbourne delays they choose the odd inbound aircraft to land on the departure runway. Aircraft are chosen tactically depending on the landing order, wake turbulence catagories and which terminal they are parking at. (Seach for Threads on "Heathrow TEAM")
Say a 747 is approaching from one of the south stacks for 27L and the following aircraft, an A320 also from the south stacks is for 27R. Intermediate Wake Turbulence rules dictate that you must either remain at least 5 miles behind or 1000ft above, but the controllers want to end up with the A320 2 miles(?) diagonally behind the 747 once they're both established on their respective final approaches. By planning ahead and placing the A320 on the other side, it saves Heathrow Final Director quite a bit of work and makes it much easier to put the A320 2 miles diagonally from the 747 infront.
This is one reason for it! I hope the explanation makes sense.