Conical or biconical perforations result (as said) from the use of bits that tend to be conical themselves. However, this effect is exacerbated by "drill wobble"—the tendency for a drill to be held insecurely, and to move laterally while it's spinning. This has been more likely to occur in ancient times than modern, and at primitive industries—such as in pre-Columbian North and South America, and Africa.

India pioneered the use of the diamond bit about 2,000 years ago. Consequently, their beads are potentially much better made than many others. Nevertheless, there are also plenty of cranked-out poorly made Indian beads, with perforations that are sloppy, eccentric, and that meet in the middle at an angle.

JDA.