Ever wondered why the 260lb sheet is actually a lighter paper than the 200lb weight? Imperial measurements of paper follow different rules to their metric equivalent, so are confusing to the unwary eye.
Imperial paper weights refer to lbs per 500 sheets of the papers size.
Metric paper weights are grams per square metre (regardless of the actual papers size).
This does mean that with Imperial measurements the weight changes if the sheet size does, for the same thickness of paper!
560 x 760mm sheet in 300g/m² = 140lb
but if we halve the sheet to
560 x 380mm in 300g/m² = 70lb
for exactly the same thickness of paper (no wonder it’s confusing)
So to answer the question at the top, the reason why a 260lb sheet is lighter than a 200lb one, is because it’s a much larger sheet. The 260lb (356g/m²) sheet is 660 x1016mm, whereas the 200lb (425g/m²) is a smaller 560 x 760mm, so the larger sheet naturally weighs heavier when 500 sheets are put on the scales, compared to the thicker paper in a small sheet size.