Question about cervical length ultrasound

I've been getting my cervical length checked every one-two weeks since I was 23 weeks. It was averaging around 3.3-3.4cm. I switched OBs at 26 weeks and the new tech has consistently gotten shorter readings. At my last (and final) check at 31 weeks it was around 2.5cm. I asked her how to tell where the external OS is because where she was putting the caliper didn't look ANY different than the surrounding area. She kinda whispered and said "Well this is where I was taught to measure from. The Dr. wants me to measure it out to here, but I never do" (and pointed to a white line that she said was the end of the vagina) That made me really curious so I went home and googled "Cervical length ultrasounds" and basically all of the images show it being measured to the same white area that my Dr wants her to measure to.

So my question is, could my cervix actually be longer than the measurements she is giving me? Since I'm measuring at 2.5 my doctor has me on strict bedrest until 34 weeks just in case.

I'll attach a picture I grabbed off google. I put and arrow where she would end the measurement instead of where the white area is and where this picture puts the external OS. And like I said, where she was ending the caliper literally looked no different. It's like she just picked a random spot and decided that's where my cervix ended. Even my husband was confused