A decision was made some time ago to follow the IOC list in order that the Records Committee, which is not a taxonomic body, did not have to make decisions it was not qualified to make. Sometimes the decisions can be a little difficult to understand for the typical birder (I include myself in this category), because we are used to seeing a particular order in books or have used a particular list in previous formative experiences, and/or because at times the change does not appear, on the face of it, to make much sense. I remember when Sibley & Monroe's initial attempts at a revised order were published some years when many were taken aback by some of the proposed changes.
As for the reasons for the changes, these are related to the science. Again, in a way that a small committee such as ourselves cannot do, the IOC can review published and peer-reviewed information and make an assessment as to whether change is needed. As to the particular changes in the most recent List, I can only refer you to the Master List on the IOC website, where the source of these changes will be referenced. Scientific advances have meant that taxonomy is pretty prone to change in recent years, and it's sometimes difficult to keep up.
There does not appear to be a strong reason for not following the latest advances in ornithological taxonomy, especially as it would put HK out of sync with the ornithological world at large. The RC has made a decision to only update the HK List once a year in order to minimise the frequency of change. The RC has also decided that for a small number of species the English name chosen by the IOC is not the most appropriate, and has instead adopted (usually via retention) an alternative; as you may imagine, this is not always a unanimous decision!
GeoffC
