When a sandbox org is created from a Salesforce.com production org, all record IDs are preserved. When you create a new record in that sandbox, the ID format is slightly different from the production ones. The prefix is of course the same, as a few things depend on it, but the first character after the prefix will be different. In a production org that character might be 3, for instance, an account's ID is 0013000000VBUP3. In a sandbox it is not going to be 3, so it would be something like 001V0000003iXtZ. In fact it seemed to be V all the time in the past (I imagined that stands for "virtual").
Using a "sandbox" designator character in ID probably helps the system to avoid ID collision more easily. Interestingly the newest sandbox I created seems to have a different designator - e. I don't know if that's a new thing - it's certainly the first time I noticed - which may have come with this new wave of added infrastructure investment, including this new CS15 sandbox instance (coming off a new na13 production instance).