To add to this, the retention period for your eVar may be set to hold the value past login... or if your system has the concept of "short" and "long" sessions like us that could also explain it... basically, we allow people to stay logged in for long periods of time.. but to access more sensitive areas, we enforce login so that only people in a short session (30 mins) can access those areas..
Basically, despite the user being logged in (and we know their internal user id), we do "initiate login" again... in those cases, we have an ID.
Understanding your site, how it works, the retention period of your eVars, etc is all critical to understanding what data to expect... and while we can suggest certain things to look for, when it comes to the specifics of your implementation, the best people to ask would be the people who created your requirements, who configured your dimensions, and the people who implemented the code.