Just, say, google some programming question, get directed to SO, create an ID and post your question. Some time later, google an OS -- or maths, literature, whatever -- question, get directed to a different SX site, and repeat the process before (or wholly without) realising they're parts of the same whole.
Possibly do the same a few more times, and hey presto, you have umpteen SX IDs.
Seems utterly plausible to me. Not you?
There was a time when SO used their own openId server/backend.
It was confusing because you had one openId stackexchange account to sign-in into something but still had to create an account on each stack and you could login from any stack but not with your email but with the openId account... for which you used your email to login... so the URL jumping around was confusing. Add one forced logging out when cookies expire or something and it was frustrating just to log in because you could be logged in into SO but not into SU for instance (despite using the same login but with a different identity).
I could log in with another openId providers but the details are fuzzy.
