That's an easy answer Henry. The Glock barrel is rifled differently than many other firearms. They are rifled in a "Polygonal" method. Wikipedia explains it like this:
Polygonal rifling (pə-lĭg'ə-nəl) is a type of rifling wherein the traditional lands and grooves are replaced by "hills and valleys" in a rounded polygonal pattern, usually a hexagon or octagon.
Polygons with a larger number of edges provide a better gas seal in relatively large diameter polygonally rifled bores. In the Glock pistol, for instance, octagonal rifling is used in the large diameter .45 ACP bore, which has a 11.23 mm (0.442 in) diameter, since it resembles a circle more closely than the hexagonal rifling used in smaller diameter bores.
http://en.wikipedia.org/wiki/Polygonal_riflingThe polygonal method they say offers better gas seals around the bullet, which adds velocity and better accuracy. The problem with lead bullets is because the polygonal rifling is more prone to fouling/leading than is a standard rifled barrel. If a barrel gets too fouled/leaded, it can and does reduce the bore diameter. A big bullet in a small barrel causes obvious problems! The problem experienced in these barrels (H&K and Kahr also offer them) when they lead up can result in an over pressured case which can cause it to rupture (or Kaboom, KB). That case rupturing problem can also result with reloaded ammunition. I've shot lots of reloads, (not in my Glock though) and didn't understand it either. But what happens is this, a once, twice fired case can and does weaken from the pressure exerted on it during firing. In a barrel chamber like the Glock that has an unsupported section (between 5 & 7 O'Clock, to enhance feeding reliability) the reloaded case can potentially rupture more easily because it's been prematurely weakened from prior use. That can be eliminated by the purchase of an aftermarket barrel that uses conventional rifling and provides a fully supported chamber.
Hopefully you can glean some information from all of this rambling! Let me know if you have any additional questions. Good luck and good shooting!