The smallest chip denomination is actually 0.001 BTC or 1 mBTC. So you can have any multiple of 1 mBTC by combining multiple chips of different sizes.

If ChipMixer allowed you to withdraw any denomination, then your privacy is greatly reduced. Depositing 0.00256 BTC as in your example and then withdrawing exactly 0.00256 BTC is very poor for privacy. If you need to pay someone 0.00256 BTC privately, then you would be much better (for example) depositing 0.004, using 0.003 BTC to make your payment and send the rest somewhere else as change, and leaving behind 0.001 BTC to combine with your next ChipMixer deposit (or donating as a fee, of course ).