I would have thought lead was more accurate as its softer nature would allow the rifling to bite more and create tighter seals.
Yes! and No!
Lead is softer and so seals a bit better, but there are lots of other factors at work too. Because lead is softer, the rifling has to "bite" deeper, to get a good grip and spin the bullet. The speed of the bullet is very important, as even with a good bore to bullet fit, and proper rifling, driving a lead bullet too fast (varies with the alloy) means the rifling will "strip" on the bullet, resulting in poor accuracy.
Look at muzzleloaders, and you see very deep rifling, compared to say a 9mm barrel. This is to get a good bite on the bullet, as well as to give powder fouling someplace to go, temporarily.
Note that lead bullet sizes are .001-.003" or so larger than jacketed bullets in many cases. This is so they have a good tight fit in modern barrels, which are intended for jacketed bullets, as well as barrels meant for lead bullets.
Its not so much about the gas seal (which is important) but about the ability of the barrel to grip and spin the bullet properly.
Both jacketed and lead bullets can be quite accurate, but each requires a different set of conditions for best performance.