Tubing with a square cross-section (and a twist in the case of Lynskey's Ti Helix OS) provides significantly more torsional stiffness than round tubing of the same dimension (e.g. 31.7mm x 31.7mm x 1.0mm wall thickess square vs. 31.7mm O.D. round x 1.0mm wall thickness) at only a slight increase in weight. IOW, round has about 60% of the torsional stiffness of square, and weighs about 20% less than square in round numbers. It's a legitimate trade-off in applications where torsional stiffness is important.

http://mnhpva.org/tech/frame_tubes.html