In some MongoDB preliminary testing, it was observed that WiredTiger was returning that blocks were modified that actually were the same on disk.
The data shows that incremental cursors returned all of the 16MB blocks in a ~90GB file. Backups copied those blocks ... noting which ones were actually unique relative to previous file system copies. They found a large discrepancy, only 16 blocks were different out of ~5600.
Investigate on the WiredTiger side.