Monsoon morning postage due puzzler

Sometimes rating postage due charges on covers and cards plumbs the depths of postal historians' knowledge.  

Consider these two lovely first issue postal view cards with postage due.

30c Mondorf Postal Viewcard

View 6 - Bridge

Prifix 89-6 Lhbk 84-6

 Luxembourg-Ville to Rollingergrund

11 Jun 1928

Rate:  35c (15 Dec 1927-1 Jul 1930)

5c shortpaid, so correctly charged 10 centimes postage due (double the 5-centime deficiency).

But ...

35c/40c Mondorf Postal Viewcard

View 1 - Vase

Prifix 93-1 Lhbk 88-1 

Esch-sur-Alzette to Bonnevoie

6 Feb 1928

Rate:  35c (15 Dec 1927-1 Jul 1930)

The 35-centime rate was correctly paid.  So why was 30 centimes postage due charged?

My theory:  The card is addressed to Bonneweg "Postfach" (Post Office Box), but no box number is given.  Possibly the recipient did not even have a box at the Bonnevoie post office.  Therefore, a 30c Poste Restante fee was charged to collect the card.

Or was the postage due just a bit of chicanery by the recipient -- Alfons Ennesch, a stamp dealer -- to enhance the value of the card?

What do you think?