Why does the Heuristic mail settings are different from Outlook plugin and Internet Mail provider?
Internet Mail provider has two options less… :-\ :
That may have to do with the fact that the MS Outlook plug-in is inside the email client and can refer to a % of the address book.
Where as the Internet Mail provider doesn’t know what email client it is using or where the address book is.
Bump 
Is David correct or there is something else? :
David’s first comment is a very logical conclusion and his second comment is absolutely correct.
Are you looking for something more from the avast team?
Fully agree…
Just an ‘ok’ of vojtech, if possible…
Well, I am not Vojtech, but …
I can’t say it better than DavidR, his answer is correct.
Thanks Chocholo 