It shows 26G used in both the "df -h" outut and in the "inxi" output you shared. Nothing strange about that.
Most likely what
@CaffeineAddict mentioned.
As a reply to that picture where you see Debian mentioned twice. The only thing I can think of is that you have a second disk in your system where you might have also installed Debian or something like that. But can't know that for sure because you cut out the screenshot of "lsblk -f" to only show that specific disk.
More likely a misunderstanding of the KDE System monitor output vs the df and inxi output because the latter two show the same amount of space used for / which is 26G.