I don't know the exact numbers for this game but in this type of game, typically you have a large number of players in the lower ranks and then as you go up the ranks, the number of active players is reduced. This means that, unless you have a large enough player base (like Hearthstone or Clash Royale, for example), the people in the high ranks will more often than not be matched down, and the people in the middle will every so often be matched against someone at the top. This will continue to happen until a certain critical mass of active players is reached such that there are always a couple of players in similar ranks starting a game.
I've made a suggestion in another post how the experience could be improved (by weighing the rewards based on your opponent's rank), but as long as there isn't a large enough player base, it is impossible to prevent imbalanced matchmaking without creating long wait times.
