Sail makers cut a constant luff curve. In order to get your sail to flow well at top, middle, bottom, the mast must generally follow the luff curve of the sail. Lets say there is a amount of luff curve your sail maker has placed in your sail (X). In order to match X in your mast you can play with your spreaders and/or your diamond tension. You can place your spreaders forward and put huge amount of diamond tension and get to that magic number, X. The result of this would be a mast that would be very stiff down low and bend a lot up higher ( NOT a constant curve). Now lets put a lot of sweep in the spreaders and less diamond tension to get to X. The result will be a mast that bends more down low than up top (NOT a constant curve). If you play with both in relation with other you can come up with a curve that is more constant (even) and therefore better matches your sail. This will allow you to get your sail to flow at top, middle, bottom. That is a good thing.

Ken