Where the premium comes from
A car insurance price is set by the carrier from the driver, the vehicle, the mileage, the claims history and the rules of the state the car is garaged in, and no calculator can reproduce that underwriting. This page therefore never prices a policy. It asks for the six-month premium printed on a quote you already hold, doubles it for the annual figure, and treats every number below as arithmetic on your own quote.
The doubling is the whole of the premium math: the annual figure is the six-month premium times two, rounded to the cent, and it is the base for the two value reads further down. Nothing here credits a paid-in-full discount, a multi-policy discount or an installment fee, so an annual premium assembled from twelve monthly payments can differ from this figure by whatever the carrier charges to let you pay that way.
Two results look strange at the edges and are worth stating plainly. With a six-month premium of $0 the page still returns the limit verdicts, because those checks compare limits with statutes and never look at price; the cost per $1,000 of cover and the miles per premium dollar both print zero instead of a division error. The premium is the only price on the page: the deductible, the discounts and the installment fees a real policy carries never enter these figures.