Usually it is because of where the first few aircraft in the arrival sequence come from before holding is required. For example, if it is rwy 30 and the first few are from BUBIN, the get short cut to Sedpo. The arrivals manager computer works out the landing times not taking this into account so this means that there is a effective 3-4 minute gap that gets created and it will mean everyone gets short cut from that side. It can be averted by doing what is called 'taking extras' but this means the Dubai guys have to see it about to happen and ask for it. Darax guys for 30 when sent on a southerly heading from PUVAL cause the same issue. Reverse all of the above for 12 arrivals.