Incomplete sync?

I have a question about what looks like incomplete syncing between emClient, and yahoo, Outlook.

I have an email account at Yahoo.  When I use the yahoo web interface, I see that there are 711 messages in one particular yahoo folder.  When I connect to the yahoo account with emClient using IMAP, emClient reports the same yahoo folder has only 510 messages. 
Why the difference?

I have made the same Yahoo email account a connected account on  After syncing, reports the same folder has only 700 messages.  When I connect to the account from emClient, using IMAP, emClient reports that the size of the same yahoo folder is only 494 messages.

Why am I seeing four different answers when it should be the same number everywhere?