Strange statistics from gparted

[Richard Owlett <rowlett@cloud85.net>]

I am experimenting with custom installs of MATE on Debian 10.7 .
My goal is to minimize its footprint on disk, and reduce menu clutter.
I have a default install with / on sda1.
The custom install has / on sda2.
Both installs use /sda3 for /home.

When running gparted from the first, it reports
  sda1 used space is 3.85 GB
  sda2 used space is 2.25 GB

When running gparted from the second, it reports
  sda1 used space is 4.77 GB
  sda2 used space is 1.80 GB

Why?
TIA ;}