If the chart is 1:80,000 that means that you have 80,000 square inches representing land & water area squeezed into 1 square inch of the chart.
But when using the latitude scale on the chart for measuring distances, I have found that there can be a slight difference from near the bottom of the chart compared to the top of the chart.
Jacdale's post was spot on.
Boasun, you probably teach others this:
Chart is a flat paper representing a sphare. It is impossible to make it an accurate presentation.
Your statement is only true around one (reference) latitude. at any other latitude it is either less or more - but wrong.
Nautical charts are pretty accurate in representing angles, but not so good in representing surface areas. That is why we read distances at the closest left of right edge of the chart at about the same latitude as the distance was taken.
If we would only measure that distance and multiply it we could not get a very accurate result.
The error is neglectable for charts representing small areas, but can be significant in a general charts like Ca4700 or US13003.