This is a result of the new message counting code in KBiff. In order for KBiff to know how many new messages are in an mbox mailbox, it must open it up to read it. When this happens, most other 'biff' utilities (including your shell's built-in one) will assume that you read your mailbox and announce it as old (or "read") mail.
There are three ways around this:
Don't use KBiff.
Use the 'file' protocol instead of the 'mbox' protocol (note that you'll no longer know how many new mails have arrived)
Convert all of your 'mbox' mailboxes to 'maildir'