In metro meetings, as you say there is better class horses, but compared to country or mid-week metro meetings, there are more possible winners because there is more class.

In winter, it's hard to line-up the form from proven wet trackers and runners with no or little wet form yet [how do we know their not a duck?]. Also worth thinking about is the problems with the slower pace that wet tracks produce [it's bad enough as it is on dry tracks]. Horses get into far more trouble in slower races due to bunching up in a mob.
Syd. also produces some very small fields and they can be very hard to pick because they are often very tactical races.

My SR also goes down in winter.