What are the reasons for the distortion on the cards?

There are a large number of cards. Of course, the maps show everything to scale. But if you look at the maps and do some calculations, then there will be significant errors, and the further from the equator and closer to the poles, the errors will be more and more. The problem is that the Earth is a ball, so all calculations will be correct on a globe, this is an accurate model of the earth. And if you transfer the surface to a plane, then due to the sphericity and bulge of our planet there will be distortions. They can only be avoided along the equator line.