R4.0.1 installer adds extra 10 GiB to Manual Partition schemes

[duc...@disroot.org]

R4.0.1, all checksums verified, GPG sig verified, GPG fingerprint
reasonably well confirmed through online searches. Installing from a
bootable USB flash drive created using /dd/. I couldn't find anything
relating to this issue after searching.

This issue is similar to the one I reported earlier but for sanity I've
posted it separately.

As before, boot Installer to the Installation Summary screen, accepting
all default options along the way. Follow on-screen prompt to
Installation Destination screen. Then:

1) Select internal 128 GB SATA SSD as sole destination drive.

2) Check "I will configure partitioning", and proceed.

3) On the Manual Partitioning screen, the Available and Total Space both
report as 111.79 GiB.

4) Create an EFI partition manually, with a size of 500MiB. Available
space now drops to 111.3 GiB, which is correct.

5) Create / (root) partition with a size of 50 GiB. Available space
drops to 51.3 GiB, which is not correct - this is a drop of 60 GiB, not
50 GiB.

So, where did the extra 10 GiB go?

As before, this behavior can be replicated across boots of the
Installer. I didn't proceed with the installation so can't advise if
this is a cosmetic issue or functional issue.