ZoneMinder will always calculate the centre point of an alarm in a zone to give some indication of where on the screen it is. This can be used by the experimental motion tracking feature or your own custom extensions. In the alarmed or filtered pixels mode this is a simple midpoint between the extents of the detected pxiesl. However in the blob method this can instead be calculated using weighted pixel locations to give more accurate positioning for irregularly shaped blobs. This method, while more precise is also slower and so is turned off by default.