What is mercator projection?

There should be a slight difference.

That is the effect of Mercator projection. We use the latitude scale (the one on the sides of the chart) to measure distance, because 1′ of latitude is the same as 1 nautical mile at that latitude. As a result, when you measure the distance between two points on the chart, you should always go to the side of the chart at the same latitude to measure the distance.

SUMAN KUMAR