Chipmixer bases privacy on the intelligence of its users.
If you pull out a private key and sweep all of your chips back at once, say you deposit 0.02{btc] and get 0.016 and 0.004 btc and then put the two chips into the one transaction to redeem them, clearly you’ve messed up as it’s easy to spot with blockchain analysis.
As apposed to this, you could use 2 addresses to send the funds to after waiting a fewdays or having the private keys and sending them to different addresses at different times. E.G. send to the mixer on a Sunday, pull out 0.016 on the Friday and 0.004 on the following Wednesday means your activity is a lot better hidden.