Let’s walk (and dogsled, and swim) from the North Pole along the Greenwich meridian (0 deg. longitude) through England and down to the Equator. Here make a right turn. That’s a 90 degree angle. Go until you hit San Salvador Island, west of Ecuador, located on the Equator at the 90th meridian (+90 degrees) . Now take another right turn (another 90 degrees) and head back north through St. Louis, back to the North Pole. So far we’ve taken two right turns, 90+90=180 degrees. But then we hit the North pole again, coming up through Canada. We find we arrive having completed a triangle. We took three turns. But you are coming in at right angles to where we left. The last angle has 90 degrees. So we’ve made a triangle containing 90+90+90= 270 degrees, not 180. Such is life on a curved surface. Even if you can’t see the curvature while moving around, it is there.