When in "Computer Strategy Mode" the Simulator looks for value and it considers your starting lineup needs. The heavy-up on QB and RB is suggesting something to you.
Also, try experimenting with your Simulator inputs, even if they don't 100% match your league. The idea being we want you prepared for all kinds of situations during your LIVE draft. That way, while the others are forced to be thinking on their feet, you'll have plenty of experience and it will be automatic.
The Simulator might be loading up on QBs and RBs because it sees the most value in those spots. If you really don't like that, tell Simulator you require more starters at WR and TE (even if that's not 100% true). By doing this, you can train the Simulator to do what you want. But, be careful... again we want you prepared for anything that comes your way on Draft Day.
Finally, try the Simulator in "ADP Mode" too.