LR on iPad "All Photos"

There is a difference because the two systems don’t overlap.

In the Apple Photos app, “All Photos” means all photos stored locally on that iPhone/iPad; and I think that can also include photos synced among your Apple devices using iCloud, if you have Photos sync turned on in iCloud.

In the Lightroom app, “All Photos” means all photos stored in Lightroom locally (but not Apple Photos), plus all photos synced through Lightroom Photos on Creative Cloud, which can include photos in Lightroom Classic enabled for syncing to Creative Cloud. It will not include Lightroom Classic photos that are not enabled for syncing.

The only time the two sets might look like they overlap is if you set the Lightroom app to automatically import photos taken by the device camera. When that option is on, the camera saves the photo to Apple Photos as it usually does, but Lightroom would then also make its own copy, and upload it through Creative Cloud.

All Photos on the iPad is all synced photos.

All Photos in LrC classic are not necessary synced to the Adobe Cloud Storage to be available to the ecosystem applications, Lightroom cloud based application and Lr mobile for smartphone and tablets.

My iPad shows 8,279 images in all photos see the screen capture from my iMac.