value session_data field in ci_sessions table is NULL - Why?

[eluser]moonbeetle[/eluser]
Hi,

I did some testing with both the OB Session library as well as the NG Session library for a user authentication. The field "session_data" in the "ci_sessions" table is used for storing user data (stuff like: firstname, lastname etc.) in a serialized way.

I noticed after a while that for some rows in my ci_sessions table, the value for the session_data field is NULL. I'm wondering why this is.

Is it because a session regeneration occured? (I set the config to 300 seconds)
Is it because a session was destroyed?
Is it because a there's a problem with the session cookie for certain users?