Marking Emails As Read

Since the moment I am using the email manager, there is one thing that comes to my attention. It happens almost every time when I do the routine that goes like this:
 
I select a couple of unread emails (lets say 5 or 6).
Rightclick
mark as read
 
Now a couple of them are marked as read, but there are still some unread (they didn’t change status).
Right click again
mark as read.
 
Same issue. It depends on how many emails I have selected wether I have to do this once, twice or sometimes even three times.
 
I just wanted to let you know. It not a major issue, but somehow I find it a bit strange behaviour. Cheers!