It's sort of a simple answer. The difference is AoE damage versus single target damage. While Shattering Light does single target damage in a line, Explosive Light does AoE damage to all enemies around your target.
You're probably missing the main part. The attack stops at your target. It's not an AoE rune. So when you kill the FIRST target, it goes to the next target after your FIRST target. Just because the rune says "line" doesn't mean it does the same damage in that line. It just means that the attack goes in a line but stops at the first target you hit and continues after you kill that target.
I think it would have been specified if so... "Wave of Light does an additional 820% weapon damage as Cold in a line".... to all nearby enemies or pierces through enemies. But it doesn't say that. Surrounding enemies aren't affected by this. That's why you'll notice that you'll have to aim exactly at your target in order to kill it.