The number of messages PR_CONTENT_COUNT doesn't match with the total number by each folder

Hi,

I'd like to retrieve the number of messages for a mailbox (Exchange 2010) from MAPI property PR_CONTENT_COUNT(PidTagContentCount). But I find that this number is slightly different from the total number by each folder.

Say I get 100 messages from PR_CONTENT_COUNT , but it is just 95 when I sum up the number by each folder.

Why is it? Does PR_CONTENT_COUNT return the number including some system or hidden messages?

Thanks,

Victor