I found some advices about anonymity, however one sentence is unclear for me
The quote comes from https://en.bitcoin.it/wiki/Anonymity
The protection that this method offers is significantly reduced if you are trying to anonymize more than about 10% of the total number of Bitcoins that the eWallet service holds. You'll end up getting your own coins back instead of other users' coins. Withdrawing Bitcoins more slowly and in smaller increments will help reduce this problem. Sending coins to an eWallet service in the largest single transfer possible will also help.
I don't get idea of "your own coins" - as far as I know there is no "coin" as an identifiable whole, just a log of transactions which balances as ones "bitcoins amount". What was the author intent to express by this sentence?