Why can't I increment partition size with cfdisk?

This is an screenshot of cfdisk, showing that I have 20G of free space. When I try to resize /dev/sda2 to just 12G I get an error saying that I have not enough space. (10197402624 bytes == 10G).

However, when I try with the "Disks" app...

It doesn't complain and everything works as expected. What is wrong with cfdisk? The same thing happens when I use fdisk.

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