Doctor keeps changing due date 😑

My doctor keeps changing my due date. It went from August 1st to the 3rd, and kept going up every time I went in. My last appointment was yesterday and now she says it's on August 15! Wth? Why does it keep going up? The same thing happened when I was pregnant with my son. They also told me that he would only be around 5 lbs. I knew there was no way, and lo and behold he was an 8lb baby.

I keep very good track of my periods, ovulations and sex. I know the time frame I got pregnant as my husband is away a lot. It is definitely not 2 weeks later than expected. Any ideas on why my doc would keep pushing it back? I need to make arrangements for leave for work and can't do that if she keeps moving the date around...