Curious momma

Okay ladies I’ve been tracking my pregnancy week by week since I’ve gotten my due date at 10 weeks and last Wednesday I thought I turned 23 weeks off what the doctor gave me but come to find out Thursday morning I had another appointment and the doctor told me I was 23 wks and 5 days! How could that happen! Is it gestational age and how the baby is growing? I’m a bit confused my due date was August 28th now it’s the 24th. This is my first baby so please no negative comments I’m just curious