After you delete an image or collection from Lightroom mobile, the image or collection displays as synced in Lightroom.
While the image or collection appears to be synced, the number of synced photos is different between Lightroom and Lightroom mobile.
Photos that are deleted from Lightroom mobile are not deleted from the cloud. Therefore, the number of synced photos could be different between Lightroom and Lightroom mobile. Use this workaround to remove the images or collection from the cloud, and coordinate the data between the two applications.