Effacement discrepancy??


Im 36 weeks pregnant and had an OB appt at 9:30am today, where the Dr checked my cervix and said "it's very thin and soft like butter", then said I could be in the beginnings of labor. Also did a quick ultrasound to check heartbeats and saw my baby A (I'm pregnant with twins) head was engaged in my pelvis which explains all the pressure I've been feeling there. My OB has me going to L&D; at the hospital weekly for NST (non stress tests) on the twins and at 4pm today the nurses noted that I was having pretty regular contractions and I told them I feel a lot of pressure in my pelvis. They had the midwife check my cervix just in case the contractions were dilating me and the midwife said my cervix was soft but thick?? Like 30% effaced. Last week I was 50% effaced. And this morning I was like 90% effaced. My twin A is engaged in my pelvis, she's not going anywhere so it can't be that she moved right?? How can 2 people get such drastically different effacement results inside of one day? I feel like either the OB or the midwife are wrong, right?