How to resolve missing storage on macOS Monterey partition in dual-boot setup?

Dual booting Sequoia and Monterey OS's on an Intel iMac; deleted all user files and removable applications on the seperate partition with OS Monterey on it but the Data showed as about 8.5GB and the remaining applications show at about 1.5GB; so a mystery 6GB. OS remained normal in it's sub partition or whatever it's called; i use exactly the same user and any other name for Sequoia so I can only presume that somehow it's picking up some portion from Sequoia; is this a known issue? Re starting and unmount/remount made no difference.

[Re-Titled by Moderator]