Every now and then I notice a gravatar associated with someone's answer but no reputation score underneath. I thought this was because the person was no longer a member of PH. But, I can click on their username and see their rep score in their profile. (When the username is unclickable I assume the user's account is gone.)

Any idea about what causes this mysterious behavior? As a mathematically-minded person I've looked for a pattern but haven't found one. For example, I thought that maybe posts older than a certain amount of time don't include reputation scores for anyone but that's not the case.

(If folks can't find examples of what I'm talking about then I'll upload a screenshot to a photo sharing site and link to it.)