Yes it could, but it would have to unpack the .iso to do that and it could increase the overall file size considerably.
They are hardly errors, just notification of files that haven't been scanned, many AVs don't even bother to tell you what files haven't been scanned, 'ignorance is bliss' as you are blissfully unaware of the files that weren't scanned.
I would rather avast didn't go down that road of not reporting but this name (which I should like to see changed) is hardly a reassurance that nothing is wrong.
There is nothing to stop you keeping these .iso files in a specific folder and excluding *.iso files in that folder, as there is no immediate danger from an iso file until it is burned to CD, etc. at which time the resident scanner would be scanning newly created files, so if there was malware it should be detected at that point.