Think of it like an eye exam. An eye doctor doesn't make everyone start at the hugest letters and work their way all the way down the chart until the letters are too small for them to read. Instead, he starts on a row somewhere in the middle of the chart, and if the person can read those letters easily, he moves to a lower row, while if the person cannot read the letters at all, he moves to a higher row. Everyone he tests in a day may attempt to read a different number of letters, and different sizes of letters, but at the end of the day the doctor will have fitted everyone with the right prescription.
Think of it like an eye exam. An eye doctor doesn't make everyone start at the hugest letters and work their way all the way down the chart until the letters are too small for them to read. Instead, he starts on a row somewhere in the middle of the chart, and if the person can read those letters easily, he moves to a lower row, while if the person cannot read the letters at all, he moves to a higher row. Everyone he tests in a day may attempt to read a different number of letters, and different sizes of letters, but at the end of the day the doctor will have fitted everyone with the right prescription.