Why does Smart update it's cache so often?

Smart update's it's cache even when there are no changes. Why does it do this?

To ensure that Smart is working on accurate channel data it has to ensure that it's cache is consistent with the local system. As other package managers and packaging systems can update the local system outside of Smart, Smart needs to verify that nothing/something has changed on the local system. For example on RPM based systems the local rpm database (which is just another channel to Smart) needs to be scanned even if the remote channels have not changed. This is done to ensure that Smart works on your system in a consistent, safe and secure manner.