Drop and light (and sometimes we've got more than the normal amount of lights) can be confusing esp at the start of a dive when a lot of stuff is going on. There's just not a good reason to the signals to be SO similar. I'll give a real life example:
Going into a restrictive cave and at 70ft there's an equipment drop area where things like 50%, scooters, stages, camelbacks, etc are staged and its also a good time to check over your buddy to make sure things are squared away before starting the dive in earnest. Buddy's backup light is on. What's easier? A quick signal and my bud fixes it or me having to get over there with lower vis, somewhat restrictive cave, lots of gear, and silty as all get out and fix it for him? The same signal is contextual for both conditions at that point of the dive because its a staging area, but also a 'check over your buddy' area.
A more simple example would be diving a long and hitting stage drop pressure, you signal, and your buddy goes to check his backup lights because he's not at pressure yet.
Is it some sort of critical thing? No, not really. Is it ambiguous? You bet. UW communication should be clear and simple as can be, imo.
SMB shoot is pretty obvi, though.