One quarter pound is the weight of the spool. Using the option to change products on the website you'll see that the line comes in a 30# weight (400 yards per spool) and a 50# weight (250 yards per spool). Notice how both spools are the same size and weight, with different lengths of filament.
Things probably seem less odd when you consider that a fishing reel is constrained to a certain volume of line; the heavier the line the less you can wind on the spool. Given that it's unreasonable to assume anyone will know how much line of any given diameter will fit on their reel it's simpler to sell bulk line by weight. There's probably some attendant cost savings with having all of your bulk rolls be the same size too.
Fishing line is marketed by breaking strength, not diameter. 50# line made from different materials will have different diameters. You should, however, be safe substituting one 50# monofilament for another. Any size differences are going to be very minor. Think aluminum foil, which may vary in thickness by brand without affecting its use in craft projects.