/rw is the mountpoint for the private volume’s filesystem containing the AppVM’s data. E.g. /home is bind mounted from /rw/home.
fstrim told the storage system in dom0 which parts the VM filesystem currently considers to be unused, so that they can be discarded.
Did you maybe divide the byte size by 1000^2? One MiB as shown in Qube Manager is 1024^2.
/rw is mounted with the discard mount option, so normally data is discarded automatically. But I think this could get out of sync in case of a crash (or maybe even a regular shutdown) right after deleting some files.
VMs also come with a systemd unit fstrim.timer that runs a VM-wide fstrim command. Ideally this should catch cases like yours where the VM’s and dom0’s view of unused data went out of sync, but it’s scheduled “weekly” and if it’s in a short-running VM I’m not sure if the timer will ever really trigger.