The terutz is that בעצם the חיוב כתובה of 100 or 200 is תלוי in her status בשעת נישואין. The יסוד חיוב כתובה is משעת נישואין. The reason יש כתובה לארוסה is because she is theoretically fit to have נישואין and to be a בתולה בשעת נישואין [which is the עיקר קובע]. So if בשעת אירוסין she was a בתולה and then was נאנסת she gets two hundred only if we assume that מזלו גרם. This way we can say that she was ראויה at אירוסין for 200 and the fact that she was a בעולה at the time of נישואין doesn't cause her to lose because it was HIS bad mazel since it happened after the אירוסין when there is already a חיוב כתובה [as we explained in the two shiurim]. THAT is why the Mishna had to add that מזלו גרם. [See שערי חיים from Rabbeinu Hagadol סימן ל"ה]
שפתיים ישק!!
Note: If you didn't understand what I just wrote - YOU ARE NOT ALONE!!!
I could explain better בעל פה. I hope😊😁
