Question about season 4

I just finished watching Season 4 episode 6 (this season is easily the best so far)

And after what happened to Miles it got me thinking about something.

If I recall correctly, right before the club went to prison they brought in Miles and the 2 other prospects all to start off as prospects. And they said the prospect period is 1 year minimum.

Their sentences was 14 months so over a year and Miles is a full club member but the other 2 are still prospects... why is that?

Is that just random and they never tell us why? Or did he start before them and Im maybe messing up the timeline?