Are these vestigial seeds in a "seedless" orange?
I'm from the garden/landscape section of the site, but post your question there anyway, regardless of my answer - you may get a better/different one!
Navel oranges, technically, are parthenocopic, which means they produce fruit without fertilisation, and that's why they are seedless. However, if the blossom is pollinated by a suitable donor, then seeds may form. These are usually, as you've discovered, vestigial seeds and may not even be fertile. The cause of the pollination (if the fruit is grown in the States, usually) is beekeepers bringing bees to orchards where other varieties of orange may be growing, or other varieties growing too close to parthenocopic varieties, usually out of ignorance by the grower when planting. Quite often, the navel fruits with seeds are of a smaller size, though not always.