[ubuntu] Warning mssge "Filesystem-root only 1.1 GB" But FileBrowser says "2.7 GB FreeSpace"

Any OS needs space to work in. If free disk space gets too small then the OS may not even load. The OS creates log files as it loads. It needs a variable amount of space to be used on different occasions. It has folders that need to vary in size and efficiency is better if the disk space is in contiguous blocks rather than scattered all over the disk. The file system (Ext4) is designed to limit disk fragmentation by reserving space for our documents to grow into. It is called Extents

https://en.wikipedia.org/wiki/Extent_%28file_systems%29

I would be very surprised if an OS did not make a claim for disk space which it will sets aside for potential future use.

The file manager knows nothing of this. It is only counting sections of the disk that have data in them and then doing a subtraction calculation.

Regards.