Polyps question

I have a question and I promise I’m not trying to sound retarded but like everyone else here I am just looking for possible answers....

I’m had very heavy bleeding for almost 6 years and suffer from secondary infertility. I’ve done a pelvic ultrasound twice, HSG test twice saline sonogram twice over the years. Well this last saline sonogram detected only 1 polyp.

I had surgery to remove it today, he said there were two of them. This was my first surgery actually using a camera and looking in the inside of my uterus and finally finding something that could cause my symptoms. Is it possible that they have always been there but have just been over passed by the other test?? I know we’re not doctors but I just wanted to see if anyone else has experienced this.