It's exactly the same as if you deposit $100 cash in your bank account, and then withdraw it again later: the bills you get almost certainly won't be the exact same bills you deposited, they'll have likely come from another depositor, or even several different depositors.
I had though of that but somehow had not thought properly of the consequences for someone who is following the money trail. The trail does get scrambled when thought of this way. Thank you for enlightening me.
Out of curiosity, do web wallets usually create one address for every user or do they have one or a few big addresses every user dumps his bitcoins into?
Edit: answering my own question, both ways seem possible, the latter especially with hybrid wallets
https://en.bitcoin.it/wiki/Browser-based_wallet
