Clearly, given which approach could be described as "unthinking" and which might qualify as "overthinking", I fell (read: fall, who do I think I'm kidding) into the latter category. So, speaking as an insider, I know there are a few different motivations that lead to this kind of careful candy accounting.
One possibility is that the kid in question has one or two favorite candies, and the kid wants to first find out how many of those favorites he or she actually managed to score, and second ration those favorites accordingly. (Corollary: if there is a surplus of someone else's favorite candies and a deficit of one's own, some reciprocal candy exchange may be brokered.) It's dispiriting to discover without warning that there aren't any Peanut M&M's left in your stash, and simultaneously realize that you didn't properly appreciate the last of them, whenever past-you happened to gobble them down. I do have my favorites, and I do try to savor them, but this isn't really my main rationale.
Another possibility is that the kid has come to the sophisticated realization that not all candy goes together perfectly. It is gustatorially jarring to chase a mini Mounds with a strawberry Starburst (or any flavor of Starburst, at that). Laying out and tallying up the candy allows the kid to, at the very least, identify the chocolate/non-chocolate divide and proceed with some kind of flavor-profile coherence. This is getting closer to where I'm coming from ...
All right, look, the thing is, I like to eat Halloween candy in a very specific order. I enjoy smooth transitions from one combination of ingredients to the next. So I like to know exactly what I have on hand in order to arrange things in their proper order. If I can form a taste bridge between a fun-sized Milky Way and a Hershey's Special Dark by way of a fun size Milky Way Midnight, why wouldn't I obey that simple logic? Or start with a Milky Way, proceed to a Snickers (essentially a Milky Way plus peanuts), follow that with a Snickers PB Squared (the "PB" is for peanut butter), and stick the landing with a Reese's Peanut Butter Cup?
Don't judge me. I can't be the only person who does this. I'm just willing to admit it.