SAT Scores

Hi guys!
Do you find that your test scores fluctuate? The first time I took the PSAT I got a really bad score (165), then I studied a little and got a 203. I started an SAT class and for the diagnostic test I got a 1690 (and it was with Kaplan!). Does that mean I just got extremely lucky for my second PSAT? I’m just confused. I think it partially has to do with the fact that I hadn’t built up any stamina to take the SAT and didn’t actually know how long it was. I could feel myself burning out about halfway through the test. Naturally I’m going to study a lot more; I’m just very confused about this.