Last logon in "server statistics" updated incorrectly

[troublemaker]

5.0.13_GA_2791.UBUNTU8.FOSS

Users access Zimbra using POP3 clints - TheBAT! and OE.

Yesterday, on March 3, I looked for accounts page sored in he order of last login time. On our big-boss' line there was a date of February 27, like he did not received any mail for 4 days.
I asked his secretary to remind him to turn on his computer: he had something in his maildrop.

But I was confused, when some minutes later his maildrop became mpty, MTA log shows tha he asquired his mail via POP3, but last login date (even after restart of the server and re-launching browser with admin panel) still remains same: February 27.

Right now (1:48pm, 4-mar-09) his line still displays that he logged on at 27-feb-09.

What does it mean?