For LVM you usually have to pre-allocate the space. So perhaps you think that you will need 8GB to hold the changes during the snapshot operation, and it works

For LVM you usually have to pre-allocate the space. So perhaps you think that you will need 8GB to hold the changes during the snapshot operation, and it works great for 6 months until more data is added and 1 customer does a lot of small updates during their maintenance window, which overlaps with your backup schedule... and the snapshot operation fails. No data is lost in this case, but the back up doesn't finish.