It has to do with how close the particles are to the lens and how close the lens is to the flash. I know that some brands make them bell shaped and diamond shaped instead of the typical round orb.
Your Sony cam doesn't have an external light and I think that is why you don't pick up many . If I don't use the external light my cam doesn't pick up that many.
Here is a good explanation about orbs & digital cameras.http://www.sgha.net/orbs.html
And below is an article that used to be on Fuji Film. I would think it applies to all brands.
Floating dust particles may cause white spots appearing at different positions on pictures taken by a digital camera using the flash. Dust in front of a subject reflects the flash light and the image of the dust is captured out of focus.
Dust, snow, rain, pollen, condensation, or any small airborne particles may cause the same problem.
The photograph will exhibit the nearby dust particle to be light and big because it is out of focus, and to be round as the shape of the aperture.
The particle will appear light and big because it is out of focus; it may assume a shape similar to the aperture of the camera, usually round.
This problem occurs when the flash emits light. If you find this spot on the playback image, to correct this problem.
Take another picture: The dust that caused the problem may be out of view. Just taking another one may solve the problem.
The newer Fuji cameras cope better with dust particles.