jet97 - not sure this is how it works...at least this is not the way it used to work.
For example, through Aperture or iPhoto you can backup your entire collection of photos (say you have 900 photos) and then delete them from your photo stream.
All your devices - iPhone, iPad, iPod, Mac and AppleTV will show "zero" photos in the stream - this is correct.
Now, repopulate your photo stream with the 900 photo backup. Since they're all "reloaded" within 30 days, all 900 photos will show on your iPhone, iPad, iPod and Mac (it's actually a cool process to see the photos suddenly appear across all your devices at once). Unfortunately, the AppleTV will only take about half (or less) of your 900 photos. In fact, the choice of photos taken by AppleTV on its photo stream is random.
This was never the issue with AppleTV before the 6.0 update (i.e., all 900 photos would have shown in its photo stream). Why the change after the 6.0 update?
In my opinion, I feel the addition of iTunes radio has limited the amount of space the AppleTV can hold within the photo stream - don't know. Unfortunately, I do know that this has been an issue for a long time. Wish AppleTV photo stream would work like it did before the 6.0 update.