A related question (How do stackexchange sites know that two usernames correspond to the same person?) says it's because the email addresses are the same.
But in my case, the accounts were identified despite differing (but very similar) email addresses.
How could Stack Exchange know that the two accounts belong to the same person?
Moderators do not disclose the methods and tools used for identifying sock puppet accounts.
The linked question is about ascertaining identity for the purpose of merging accounts as opposed to detecting sockpuppet abuse. The criteria for merging are stricter for the following reasons:
If accounts belonging to different people were merged, this would be a privacy issue since one of them may now have access to private information of the other. Also, merging is difficult to undo.
A false positive for a sockpuppet abuse “only” results in things like suspensions and vote invalidations, which can be easily undone. Also, it’s often impossible to distinguish meatpuppet¹ and sockpuppet abuse, but since both are not allowed and lead to the same consequences, the distinction is not very relevant. By contrast, merging meatpuppets would be problematic.
Also, the linked Q&A is more than ten years old and moderators cannot merge accounts anymore.
¹ A meatpuppet is like a sockpuppet, only that the second account actually belongs to another person whom the puppeteer tells to vote up their posts, etc.
