The bowling is calculated dynamically so it can change from game to game depending on how the bowlers are performing. I've had a few games where 5 bowlers each end up bowling 10 overs. The bowling rotation isn't perfect by any means, especially when it comes to something such as getting the part timers out of the way. It's better than it was before (in the T20 engine) because the sim now keeps count of specialist resources before determining whether to bowl the part timers or not (it tries to fit them in between the 25th and 40th overs). However, there are no calculations to make sure that those specialist overs can be bowled in tandem (alternating, etc.) as I haven't figured out an easy mathematical way to represent that.