Paul's right. Take a look at the single-end traveller when under load. You get some deflection in the line under load, no matter how tight you pull it through the cleat. The effect is that you are ALWAYS travelled down an inch or so. Now look at a split-tail - by using two attachment points, of which only the weather side is fully loaded, you have taken that inch back, allowing you to truly have the main centered. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Make sure that you have the split tail tied correctly - I often see that people tie them the wrong length, which eliminates the benefit, effectively making it like you don't have one. Tie it and test it on the beach. Reference marks on the beam are very helpful.