Quick Question [Reposted]

As far as I know, I am supposed to get Hextech chests if I do well in a match, correct? However, In this link provided, all I got was a key fragment and no chest. May I inquire as to what I did wrong? Is it a random chance of getting a chest or key fragment? Did I mess up my understanding of how to obtain chests? I do have all 4 currently available.

Thanks, ~xChamp