There's a few reasons why rosters have been fluid, revolving door if you like.
It's a World Cup year, and several blokes have been called up for Wallaby/Extended Wallaby Squad duties.
Several players have landed Joker Contracts in the NH after being spotted in Grade Rugby or early NRC games.
Injuries. Team Rehab Selectors have been very active this year.
Rest. RUPA contracts only allow a certain number of matches to be played each season (think it is 30). It is a long year for the top level Pro players especially those with Wallaby committments.
Being World Cup Year, we lost about 10 players to Barbarians Club invitations for RWC warm up games.
Rest. The Western Force Players travel a fair bit during Super Rugby. Perth Spirit (underwritten by Force) usually rest their pro players for their away games. As an aside, their home performances this year (with pro players) has not been stellar, and they have won a game or two while on tour.
Form. Being a development comp, some club players are given a shot at the big show. Some don't get invited back if they are not up to it. Selectors don't always get it right first time around.
Suspensions. About a half dozen blokes have been served up with a Citing Bloke suspension of varying lengths following match video reviews.
All that being said it has been a little frustrating trying to tip the bloody NRC because apart from City and Vikings, performances have varied wildly from week to week in many teams. The Stars started with a twinkling array of rock star players, and got some impressive results before most of these departed the scene leaving them looking and playing like Stars version 2014. Having got early runs on the board they have been rather poor, compared to teams like Eagles and Spirit who seemed to improve as the tournament went on, only to fall agonisingly short of finals qualification. Their active rosters at the end of the season were probably superior to those of the Stars finishing roster, but the competition is for 9 weeks with all games counting before the knockout starts.
Anyhoo, I think all this will be academic as the City and Vikings will march on to the Grand Final leaving the Rising and Stars in their wake like Semi Final road kill.