It's due to the manufacturing process. They are aluminum extrusions - they come out of the extrusion die quite hot (and horizontal due to space constraints). They try to support them as they cool, but you can expect a 40' mast to be out of straight by an inch - perhaps two. I'm not aware of any extruders that roll them to try and keep them straight.
When setting up my mast, I first get the masthead the same distance from both toe rails. I use the main halyard and a fish scale - get the same pound loading on each side to make the hook touch the toe rail. Then I sight up the mast and adjust the lowers to straighten the mast. I recheck both, then finally fit the chocks at the deck. Turnbuckles at this point are just hand tight. Final tightening is done underway. Tighten uppers equally (and lowers equally) on both sides so that the leeward shrouds are just coming loose at about 20 degrees of heel.
A tension gauge might be useful for the backstay - mine is hydraulic and I've got a gauge. Tighten the backstay, you tighten the headstay (masthead rig), and you flatten the luff of the jib so you can point higher. At a certain point the luff of the jib becomes a board and the helmsman no longer has a 'slot' for steering. On my boat this happens at about 1300# on the backstay.