It is the way the map is produced. The one in question is called a Mercator Projection. It is popular but is distorted, especially at the Poles.
The unique feature is you can take your starting point and destination, draw a straight line, then use that single compass heading to get to that destination. The issue is you will not actually be flying a straight line if you looked at the route on a globe.
The course provided in the article looks somewhat like a polar Great Circle route which actually would be the shortest distance and would look like a straight shot if placed on a globe but compass heading change while running a Great Circle route.
Government maffs
It is a straight line. It appears to be a curve when the globe is projected on a flat surface. Take a piece of string on a round globe with one end on New York and the other on Singapore. It will show that same route as projected on a flat map.
It’s sad how many people are ignorant of basic geometry principles. The Earth is a sphere. The shortest distance from NY to Singapore would be to dig a hole between the two points. However, for a long list of reasons, its better to travel approx 10,000m above sea level along the surface. And thus a trip from New York to Singapore should go over the Artic region of this planet.
Lol, the lfight is in a straight line, but it’s not flying over a map.