We only calculate unread counts for items less than 30 days old… if you’ve waited long enough that your unread items were older than 30 days… we won’t consider those older items when figuring out unread items. They will still be there, but we won’t show them as unread. This is standard behavior in Reader.

Brian S. regarding Google Reader 

I can't count the # of times this has bothered me.  Perhaps it's time to find a read news reader.  
It's probably actually saving me time, but it still rubs me the wrong way every time it happens.