I always assumed the relatively early dates were a combination of the attraction of the island to migrants and the high levels of observer coverage - regular migrants probably also pass through the rest of HK outside the main period of passage, but these are not detected because they are present in such low densities. This is similar to the fact that rare migrants are more often detected on Po Toi than elsewhere.
There may also be an element of whether or not birds are reported elsewhere in the same way - for example, it seems that Grey-backed Thrush and Manchurian Bush Warbler are not yet on Po Toi (or perhaps this week). I have been seeing small numbers of these in the Northern NT but have not been reporting single birds to Birdline or posting on the forum.
