Ratings are based on the scores of the players who played the event. Nothing else. The only difference between unofficial and official ratings will be that scores from R1 & R3 will be combined and R2 & R4 will be combined to produce the official ratings so that players will get the same ratings for the same score on the same layout. Discs do not fly as far in the winter (colder temps) as the summer so essentially the course plays longer in the winter despite the foliage. That may be the simple difference of approximately one throw in the SSA if that's how it comes out. Remember, one shot is only about a 2% difference in SSA.
Rater of the tossed arc.