The Big Sur installer creates a second volume in the APFS container at the start of a new installation, so when you erase and reformat a disk, you need to delete extra volumes using the +/- icons. If you don't, you get the situation you describe.
First, I would make sure to have made a Time Machine backup and a Bootable USB installer, before either starting again by a complete erase/reformat/install, or maybe first try just deleting the redundant volume, which I am guessing is the Data - Data one.
An additional complication I found, is that Disk Utility sometimes does not refresh its display as one might have wanted. Quitting and re-opening it finally showed the actual disk state.