Who reads Pap smears? Did you ever think about it? It's not an automated machine that looks at it. It's a person who decides what the test shows.
It's not like just anybody does this stuff. It takes training, and how well someone prepares the specimen can influence what the result is. This is why a good lab works hard to minimize bad slides--if they have a bad slide, they may have to label it as "ASCUS" when there is actually nothing abnormal.
Labs have a head pathologist, a doctor who has extra training in reading tissue specimens. Any time you have a biopsy, that tissue will go to a lab, and a pathologist will look at it. Most labs have cytotechnologists who work with the pathologists. Since the vast majority of specimens turn out to be normal, cytopathologists usually take the first look at a specimen and have the pathologist look at the ones that seem abnormal.
Once your gynecologist swabs your cervix with that huge Q-tip, it gets stuck into a tube, and that tube is shipped off to a lab. Once at the lab, someone there takes out that swab and wipes it onto a piece of glass, called a slide. Then they look at it under a microscope and determine what they see.
So right off the bat, there are two possible problems: how good a specimen your gynecologist gets, and how the lab handles it. Let's say both of those steps were done well, then you should get an accurate result. But let's say you got a Pap smear during your period--the result may be affected by the blood because the cells shed from the uterus are different than cervical cells. And if your doctor scraped too hard, the cells may be damaged and look abnormal when they're really normal.
If you do get an abnormal result and have a biopsy, that biopsy should go to the same lab as your Pap smear. Then the pathologist can look at both your Pap test and biopsy and make sure that they make sense together.
While there is science to interpreting a Pap smear, there is also judgment and experience. Before I had a cone biopsy, I had my LEEP specimen re-examined by the pathologist at the hospital where I was going to have the cone biopsy. He agreed with the first pathologist, but at least I knew that information. Then I had no doubts about the result.
So the answer to who reads Pap smears is not a machine, and there can be errors, which is why a biopsy may be really helpful to figure out what is going on in your body.