The difference is that clouds, blobby surfaces and tube are software rendered, with your RAM fully, because their shape must be calculated in a way other than just calibrate their center point. The other particles that can be rendered with your hardware, do have a center and an 'all the same shape', wich cause them to be harware renderable. I don't see why maya couldn't make the hardware renderable particles be rendered with sofware render also... but they must have had a reason