Was there a foul situation in the game? Or perhaps the others on the depth chart were in the game at other positions (unlikely since we're talking a C, but just in case...). The only time I've seen it is when fouls/injury impacts the depth chart to the degree that the Sim literally has to grab a random player to serve as a backup at the position (in my case it was a PG playing PF...).
To answer your second question, yes the depth chart still is important for target minute -- the players toward the top will be subbed in to try and achieve their minute range before those in the 3/4 slots. I've heard it said there is some difference in how the "give more time to backups" command is executed between fatigue and target minutes, but I'm not sure if that's fact or forum rumor.