Go into a folder and select a message by clicking with the mouse, then hit Shift+End - works as expected. Now do the same but jump to the message where you want to start with by using the arrow keys on the keyboard then hit Shift+End again - you'll notice that it selects more messages then you wanted. Try those two methods above with Shift+Home as well. Mouse works, keyboard doesn't work at all, just jumps to the top.
I just tried and was able to reproduce this immediately. For me, when I use Shift+End or Shift+Home, it starts from the email that I last selected with the mouse, even after I've moved the selection with the keyboard. It looks like the keyboard arrow keys move the cursor, but they don't update some other selection, and that's what the Shift+End and Shift+Home are based on. If it's any help, I have "Open message when selected" set to "Never."
Also, Shift+Down and Shift+Up work properly--based on the last selection (keyboard or mouse) not just the last mouse selection.
It actually marks the selection starting by the clicked line not the line where the focus is after cursor-down key. This seems to be related to Bug#1959 (and both still exists in Win7 3.9.3)
Shift+Home/End has been fixed long time ago, and now other similar selection key combinations are fixed as well.
*** This bug has been marked as a duplicate of bug 1959 ***