When playing CSGO in casual, players can sometimes receive item skins, cases, or graffiti at the end of the game. I've noticed that players at the top of the scoreboard don't necessarily get the items, so it's not based on xp. What I want to know is how does the game decide which players get an item, and how rare that item is?
Once per week, if you level up you will get a random graffiti or a skin from one of the map collections (Inferno collection, Vertigo collection, etc). Each game, except for the one you level up in and get your drop, you have a chance to get a case drop favoring more recent case collections from my experience. You can only get one case drop and one skin/graffiti per week and the week resets on Tuesday in the evening (United States).
To address your specific question, as you noted, your score has nothing to do with the probability you get a drop or the quality of the drop you get. The only time you get a drop for sure is your first level up of the week. This drop will not be a skin from any case, cannot be a Stattrak weapon, a knife, or a covert (red) item. I don't believe the skin comes from the specific map collection you played on. In other words, the chance of getting a case drop and whether or not you get graffiti or a skin from level up is completely random to the best of my knowledge.