Practically all of the coastlines of the Philippines were imported from a crude coastline shapefile generated from SRTM data back in 2007. The problem with SRTM data is that it has a resolution of about 90 meters and since the data is in a raster format (i.e., pixellated), the coastlines therefore appear as jagged "sawtooth" shapes.
In areas that have high-resolution satellite imagery available, such as in Metro Manila and Davao City via Yahoo!, the coastlines have already been refined and corrected to match the imagery. But in other parts of the country, the lack of high-resolution imagery, the generally low priority attached to coastlines (compared to streets and POIs), and the tediousness of correcting them has led to the sawtooth coastlines persisting for years since the initial import. Furthermore, some smaller islands and islets don't have coastlines visible on the SRTM-derived coastlines or are merged with their larger neighbors. There have been sporadic efforts to refine and correct these coastlines in the intervening years but majority of the coastlines remain uncorrected at the start of 2010.