I registered all the projects with the same e-mail, but they generated different CPID (Cross-project ID) for me, and my stats are seperated. But https://boinc.berkeley.edu/wiki/Cross-project_identification says “Accounts on different projects are considered equivalent if they have the same email address”, which is obviously not the case I've encountered.

This problem didn't happen yesterday, but it happened today.

Read this primer on split CPID's.
https://steemit.com/gridcoin/@dutch/the-definitive-guide-to-fixing-a-split-cpid-gridcoin-s-and-boinc-s-1-problem
You can also avail yourself of the <external_cpid></external_cpid> parameter in client_state.xml.

This parameter is in every project section in client_state.xml. A simple copy and replace works easily.

If you set that to the CPID that you want to standardize on for all projects, then the individual host_cpids become irrelevant at your projects.