Questions, questions

Hello everyone! Can someone explain po ulit in layman’s term, bakit nag babago yung EDD if 3rd trimester na(30 weeks??) Naging earlier po sya ng 1 week than the original EDD nung mga previous Check ups ko. Nadagdagan rin yung Average Ultrasound Age ko by 1 week, from dun sa normal bilang ko. In theory dapat pang 30th week ko pa lang today. Can someone explain this to me??? Sorry po, curious mom!! #firstbaby #pregnancy