CPUSE upgrades work like this:
1) Create a new partition
2) Clean install the new version (so you get a fresh file system without any leftovers)
3) Copy over the previous configuration and upgrade it
The previous partition is automatically kept as a snapshot, so you can easily revert on crisis. A small "quirk" is that the date of the snapshot is the creation date of the partition and not the date that it was converted into a snapshot. This does not mean that it is corrupt.
Also note that HF / Jumbo installations are not upgrades and are installed onto the existing partition. This means that you can uninstall them and keep all configuration changes since the installation time. On the other hand, you don't have an automatic snapshot.
In the past, minor upgrades worked like HFs and were installed directly into the existing partition. After much deliberation, we decided that the major upgrade mechanism is more robust and useful, so minor upgrades (such as R80.10 => R80.20) also use the new partition mechanism.
Hope this technical background was helpful and also explains why R80 => R80.10 did not automatically have a CPUSE snapshot.