A week ago I backed up the MacBook Pro to its Time Machine drives. After that was done, the available space went from 95 GB to 121 GB.
Today after doing the same thing, The free space went from 118 GB to 127 GB. The confusion is caused by the APFS local snapshots that are taken hourly, daily & weekly. They are hidden from the Finder & Spotlight searches and are only made if the system has available space.
Not sure why there’s an extra 6 GB of free space this time, since the last backup session, I added an extra 1.8 GB of additional essential apps.