iPhone Backup versus Backup During Sync

What is the difference between the backup that is part of the Sync process, and choosing the "Back Up Now" button under Manually Backup and Restore?

ALSO, why do the storage usage values in the bar at the bottom of the iTunes iPhone page show different numbers after a Sync and a "Back Up Now"? That is what is happening?

The usage values for every category are significantly smaller after the "Back Up Now" process than after a Sync. Only after doing 2 or more cycles (Sync, Backup, Sync, Backup) do the value align. This would seem to imply that just doing a Sync does not do the same backup, AND two or more cycles are needed to Backup everything...

While I have an iPhone 8 now, I have noted this with previous models and iOS versions (iPhone 3GS; iPhone 5).

It would be great to get some real answers, and not just guesses or speculations...