Ok, I guess that makes sense, but what I still do not understand is that, this is not the first time I have checked signature with gpgv while have downloaded an iso. I usually save the output of the check to a text file, and just dug up the last time I did this same thing, about a year ago, then while I had downloaded ubuntu-mate-16.04.4-desktop-amd64.iso. Back then the output was this:
nick@mycomputer:~/Downloads$ gpgv --keyring=/usr/share/keyrings/ubuntu-archive-keyring.gpg SHA256SUMS.gpg SHA256SUMS
gpgv: Signature made to 1. maaliskuuta 2018 21.44.02 EET using DSA key ID FBB75451
gpgv: Good signature from "Ubuntu CD Image Automatic Signing Key [email protected]"
gpgv: Signature made to 1. maaliskuuta 2018 21.44.02 EET using RSA key ID EFE21092
gpgv: Good signature from "Ubuntu CD Image Automatic Signing Key (2012) [email protected]"
nick@mycomputer:~/Downloads$ grep ubuntu-mate-16.04.4-desktop-amd64.iso SHA256SUMS | sha256sum --check
ubuntu-mate-16.04.4-desktop-amd64.iso: OK
So back then, I got two "Good" signatures and there was no problems regarding "Can't check signature". And because of this I am now unable to understand why in the past I always got no errors and now suddenly there is an error message, although I am doing the exact same thing? I guess it is nothing to be concerned about, but I do not still quite understand what has changed and what causes this difference in the output compared to the same thing done earlier?