Read User Mail module shows (10.27 kB in 1)

When looking at mail accounts using the Read User Mail module in Webmin for accounts that have received mail and then deleted all their mail Read User Mail module shows (10.27 kB in 1) for those accounts even though when clicking on that mail account there is no mail.

For some of the accounts the number is different such as (16.53 kB in 1) or (16.98 kB in 1).

If I send mail to one of those accounts the number goes up and then if the mail is removed it goes back to that same number rather than going back to nothing.

Is this a bug? What's going on?