Question

It means there was one and only one specific sequence that got amplified in the PCR. The same should be true for your DNA. Why did you get exactly one amplified sequence? Sometimes, for a very good reason, there may be more; some PCRs with one pair of primers produce two distinctly different sized products. What is one situation in which that would be expected to occur?

Answer 1

Primers are never 100% specific so they can always bind to a non-desired region of your template DNA, which would lead to a product with a size that's different from the one you are expecting. The lower you PCR hybridization temperature is, the less specific you reaction will be because the primers will bind more easily to DNA (but if you increase it too much, it will never bind).

Moreover, depending of the way you purified you DNA template, you can have more or less accessible regions (covered by proteins for example) so you could end up with different patterns of PCR products even though the DNA template is in theory the same from one stock tube from another.

You can have multiple products for several reasons:

multiple priming sites in different parts of the genome - very common for related genes

your gene of interest is segregating an insertion or deletion mutation

you are seeing PCR artifacts like primer dimers....