What are the measurements that are taken after an eye exam?

You hold up this binocular-like device, look at the green dot at the bottom and I presume that they are measuring the distance from iris to iris. However, she didn't tell me to look at the green dot at the bottom until after she had already made some measurements. You can also see their eye(s) at the top through a lens.

I was wondering because at my previous eye appointment at the same doctor, I got an eye exam and came in with my old glasses. They were about 6 years old because I mostly wear contacts. They gave me -6.75 glasses in each eye. When they came back, they were extremely blurry. When I returned, she told me that, "Maybe we were a little overzealous on your prescription." She was then going to make my glasses -6.00, which is what my old pair (that I can't see as well with) is. When I told her that I can't see as well as I used to out of them, she went to -6.25. Realistically, I felt like I was having to tell the eye doctor what my prescription should be and she seemed clueless. :-\

I think their measurements at the end after the exam (I didn't have a second exam) were also wrong, because the eye doctor told them to do my measurements again. So, I was wondering what this device does.

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