difference between free and available

You could have an ext4 partition with almost 70 gb free (i.g. on KDE partition manager) but only a few more than 10 gb available in Dolphin.

Indeed free and available are not the same: free means the total space (available for root), available (in Dolphin) is the space for a single user.

If that partition is not a root one (=is not /) you could do something like:

sudo tune2fs -m 1 /dev/sdxx 

• in the code above replace sdxx with the real name of your partition (you can know it, i.g. doing df -h), that is something like sdb8 or sdc6.
• Be careful! Make sure that you have enough linux knowledge to do safely this kind of command. At your risk.

After your command, you should see that in that partition your user-available space is only 1gb less of the total space (that is 70 – 1). If your command would have been

sudo tune2fs -m 3 /dev/sdxx

Your user available space would be =70-3.