Why burning ISO of a multisession disc results in a disc that doesn't have multisession?

[Foss User <fossist@gmail.com>]

1. I used Brasero to write a disc with this option enabled: "Leave the
disc open to add other files". Let us call this dvd1.

2. I created an .iso image of dvd1 like this: mount if=/dev/cdrom of=dvd1.iso

3. I opened Brasero again, and selected "Burn image" and tried burning
dvd1.iso into another DVD. Let us call this other DVD as dvd2.iso

Observations:

1. I was able to import session and add new files to dvd1 later with Brasero.

2. I was NOT able to do so in dvd2. When I inserted dvd2, Brasero
never showed this as a media to which I can wrote.

Why?