This is due to making the sensor area used (selected by engaging the right digital zoom) to the same size as the final image. If you "digitally zoom in" you are only using part of the sensor. If you match your final image size to the same pixel proportions as the digital-zoom is using, then the camera doesn't have to spend any time at all on downsizing or up-sizing the image data. Thus, the increase in continuous shooting rates.
You will have to do the math to find out what digital zoom is needed for a particular final image size. Or take a sequence of images at various digital-zoom factors and then read the EXIF data in the MakerNotes section of "ZoomSourceWidth" to see what resolution (width) you should choose as your final image size.
If there was a way to enable digital zoom at any optical zoom setting, meaning, you don't have to use up all optical zoom before engaging digital zoom, then you could take advantage of this speed-up at all optical zoom settings.