Why are the ratings for the same score thrown on the same course layout not the same either in different rounds or different weeks with the same weather conditions?

The unofficial ratings for each round are calculated from only the scores the propagators threw that round. Their scores will naturally vary from round to round even when it looks like the weather conditions are similar either on the same day or even the next week with a completely different set of propagators. The typical variance in a round rating for the same score under similar conditions can range up to 25 rating points (about 5%). If it's more than that, it's possible the weather was significantly different or perhaps the TD did not set the course layouts properly when uploading scores. The good news is that when the official ratings are calculated, the scores of all propagators playing the same layout in more than one round are combined so that everyone will receive the same official rating for the same score in all of those rounds.