Basically when you start out (especially in casual) you're matched in a pool only with other new players. There's a grace period of this for a few games then after that the matchmaker will start matching into the normal player pool, and it calibrates your (casual) mmr from your starting 5-10 matches or so. Same with ranked, from ranked 25-22 especially since it's halfway into the season, you're probably getting matched with other new players and kids with basic or limited cards. Once you get to rank 21 you start getting matched with people from rank 20, which is where all the people who are not new are, and they have a huge collection of cards and decks already. At rank 20, you're facing people who don't play much, or who stay at that rank and farm people for easy wins or golden heroes, or people who played for a few months and don't have a great collection yet and are stuck at rank 18-20, or on occasion really bad casual players but they all should have a decent budget collection or decks. Basically you're out of the newbie hands free zone now and everything's free game.