Ok looked at the iPad forums and looks like iPads ,some, are having similar issues.
here is one post about this issue,
“For clarity, has the attempted software update completed successfully?
When installing a system update, the [compressed] update binaries must first be downloaded (consuming storage space), then decompressed and verified (consuming more storage space) - and finally installed. Once installed, the installation binaries are deleted.
If your device runs-out of available space during the initial download and verification phase, the installation process will stop - but the downloaded binaries will remain.”
since I preformed a flawless update to 15, did not run out of space or anything affecting the
Install(that I know ). I am guessing the downloaded binaries have remained and maybe that why thinks out of space, maybe.
do have over 20 GB free and sadly a factory reset/restore is really not an option as have many playlist and mp3 that not allowed to move from iPhone back to iMac (was able to do this under iTunes and gave over 10gb of music )