If everyone thought the earth was flat until Christopher Columbus discovered America in the late 15th century, how is it that some terrestrial globes were made before then?

Before answering this we need to address two of the "facts" presented in the question itself, because although they are widely accepted, they are nevertheless false.

Discovery of North America

Columbus was definitely not the first to discover America, though he was probably the first to spread the news about the New World throughout Europe. Even if we disregard the Native Americans who, most scientists believe, crossed over into North America from Asia during the last Ice Age, there have been many parties taking credit for the initial exploration and some claims are still coming in.

Among those to announce they were the first and when include the Hebrews in around 1000 BC, members of the ascetic Celtic Church in 770 AD, the Vikings in 990 AD, Basque fishermen, Africans, the Portuguese, Japanese, Amerigo Vespucci (thought to be the person America is named after), and others. The most recent claim, that it was the Chinese, was made by Gavin Menzies in his book "1421: The Year China Discovered America," which was published in 2003.

The truth is, we don’t actually know who first discovered America and we’ll probably never know due to conflicting information and lack of scientific evidence.

Flat vs. Round Earth

Contrary to popular belief, not everyone thought the earth was flat when Columbus set out on his first voyage. As a matter of fact, Greek philosophers proposed that the earth was spherical in shape back in the 6th century BC, though it remained just a philosophical concept until the 3rd century BC when a new geocentric model of celestial motion was introduced into Greek astrology.

Encouraged by Plato, a young mathematician called Eudoxus of Cnidus proposed that the heavenly realm was spherical in shape and consisted of multiple, rotating spheres that surrounded the earth, which itself was motionless and spherical in shape.

To Aristotle, Plato’s prize student, spherical earth wasn’t just a theory -- it was definitive. Aristotle provided valid scientific arguments about lunar eclipses, stars, etc. to support his conclusion, but the one I like best was just a simple observation. He noted that when a boat sails away from you, no matter in which direction, its hull disappears from sight before its sails do. He concluded this must be due to curvature, so the earth must be a sphere.

Aristotle knew the earth was round as he watched these boats sail into the horizon nearly 2,000 years before Columbus ever set sail on his voyages.

It’s noteworthy to mention that Aristarchus of Samos, Greek astronomer and mathematician, presented an alternate, heliocentric model which proposed that the sun, not the earth, was at the center of the universe, but this concept was not well received. It was disregarded for almost 1,800 years until the heliocentric theory was revived by Copernicus and modified by Isaac Newton and Johannes Kepler.

World Globes

Although the art of making terrestrial globes became popular, even extremely fashionable, from the 16th century, some were made long before then. You may think that since the Greeks were the first to entertain the idea of a round earth, maybe they were the first to make a globe. You’d probably be right. As far as we know, the first terrestrial globe was constructed in 140 BC by a Greek named Cratos of Mallos in 140 BC.

So even if there were people who believed the earth was flat until the 15th century, though less than we may think, there were also many who knew otherwise long, long before Columbus didn’t discover America.