why are there 600 photos on camera roll but only 300 in photo stream?

if photos upload automatically to the photo stream, why do I have twice as many photos on my photo stream. It seems also to have stopped uploading photos in early March. I also have 2 photo streams on my ipad (which is synced to my phone & computer); one with 400+ photos & one with 200+ photos. I ipad also has only 300 of the 600 original photos in the camera roll.

This is confusing.