At the end of the last season in Hearthstone, it said that that I had reached a rank of Shieldbearer (20), but my chest had the number 17 on it. Does that mean I got the 17 chest because that was my best rank in the previous season, or is your chest determined by my rank at the end of the season?
Whenever you rank up, your chest improves. When you rank down, it still stays where it is. At the end of the season, you open your chest and the rank it has will reflect the highest rank you achieved during the season.