I'm learning to boot a live distros and noted in one that casper folder contains two sets of vmlinuz and initrd, one like vmlinuz and other vmlinuz-4.xx-something. Interestingly when I open iso file of distro with archive manager vmlinuz-4.xx-something shows size zero, vmlinuz is not zero, when I mount iso file, both files are not zero and same size. grub.cfg menu entry have vmlinuz-4.xx-something file.

What could be the reason to have two files? (some redundancy, should ask those who made the distro?)
Why archive manager sees one file of size 0? (glitch?)

probably you can found already: vmlinuz is just a link to vmlinuz-XXX

But neither file manager nor ls -l command says so for neither unpacked no mounted folder. Glitchy distro on which I work?

ADDED: now that I paid attention to detail I know about some number of links in ls output, which is different for folders for vmlinuz but same for vmlinuz inside folder and same for filesystem.squashfs across folders...

Just by logic I would guess number 2 means these files have links to each other, but that is for folder where there are both non-zero. And info about ls said its output is implementation specific...
Final guess - don't use GUI archive manager to unpack system files even if it opens them? (or participate and fix source code)

I don't know, but probably there was an option to follow symlinks (or something similar).

The vmlinuz link is a safety measure if you have boot problems. In grub console you can boot to just vmlinuz as you can't always remember the full file name, vmlinuz-xxx.
If you click on a link in file manager it will take you to the original file so it always shows as if it was the original file.