Deleting multiple mails

Referrering to KDE 4.3.2 and KMail 1.12.1:

when I try to mark several email in the Inbox using the pointer and 
"shift" button I often get unexpected results.

An example: I am on the top email, I move the mouse pointer to an email 
near the bottom of the email list, hold down Shift and click on it. My 
expectation is that the the emails from the top "active" mail down to 
the one indicated will be marked but often it is the ones from "the 
bottom" of the window and UP to the one I am now clicking on that gets 
selected. I get different results almost every time unless I mark the 
whole visible window.

This refers to unread emails. If one or several in the visible list have 
been read I get even more erratic results.

And I am not referring to the folder list but the "mails in folder" 
window whatever it is called.

Is this the result of some settings, intentional or a bug of some kind? 
Anyone knows?

Kind regards,
Sinclair