It's fairly typical. Nails get dropped, nails are struck poorly and go flying.
Nails are not hugely expensive (modern era - supposedly in the era that they were hand-made one at a time, folks would burn down old houses for the nails, though I am dubious that that is an accurate claim.)
Carpenters are fairly expensive and not fond of playing janitor. If the crew has a less expensive person who is supposed to clean up, it's still hard to find every lost nail. So long as they don't end up standing on their heads they generally rust quietly away after a few years without causing problems.
If you prefer to get more, you can hire someone to use a very strong magnet to sweep the area - but even a very strong magnet won't pull in nails from several inches away, so the ground-sweeping operation needs to be very thorough to come close to getting them all. You can rent "magnetic sweepers" that roll along and cover a fairly wide swath, but they work best on hard surfaces, not grass. I use a few hard drive magnets taped to a discarded golf club ;-)