Help??

Hi i went for an ultra sound on the 24 of this month because I thought I was about nine weeks. they had to do an internal scan as they couldn't see anything in the abdomal scan so all I could see was a sack and looked like a little peanut in there and was measuring about 5 weeks and 4 days I couldn't  see a fetal pole. A week later I went back and seen a fetal pole but they told me i was still measuring the same weeks as the last time, but my baby looked like a little baby not a peanut but at this time I was bleeding so they were certain that my baby hasn't grown I'm just a bit confused how I didn't see a fetal pole the first time but did the second time and they told me I miscarried, can anyone help or try to explain why this is? X