Same reason as why some chips originally purposed for casino use are stamped, drilled, and notched. Except not really.
Chipco would sometimes use an actual casino chip image on one side with the Chipco logo on the back. Since the image on the casino side was the actual image submitted by the casino and could be passed in the wild as a negotiable casino cheque if an employee didn't review both sides, those samples would need to be drilled or notched.
For the chips above, if both sides are the Chipco logo, in that case I'd have no idea. Practice, perhaps? Boredom? *shrug*
To prevent counterfeits. This video doesn't explain notching, but is a good documentary of two guys who were successful at casino chip counterfeiting. That was until on guy told a girl about the scam. IDIOT! This is only a snippit. The full video I can't find at the moment.