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Do you mean backed up or do you mean in photo stream.
Photostream in the cloud only keeps photos from the last 30 days (subject to a 1,000 photo limit). When photostream syncs the photos to your device they will stay there until deleted, which means you may well have more photos in your photostream album on your device than there are in photostream in the cloud.
The consequence of this is that if you turn photostream off and on again on your device (or enable it for the first time), it will only be populated with photos from the last 30 days, which may not be all the photos that were previously in the photostream album on that device.